This is my default Bard build, and was heavily inspired by the one and only Polypuff. Unsealed spellbook can be incredibly powerful if you know when and how to use it. This build may be hard to play or to get used to if you don't typically use many active items, or don't usually pay extra careful attention to your team. If you're just learning Bard, I don't recommend using this build, and instead going with whatever is meta right now, or just having some good fun with full AP.

Bard has a very unique playstyle compared to a lot of other champions, as it is encouraged to roam and impact every lane whenever possible. That isn't to say that you should abandon your adc entirely, but when things have calmed down bot, and your mid laner can't quite secure a kill you might as well make your way over to mid. You have to pay attention to every lane and the situation in each of them as well in order to maximize your roaming efficiency. Be careful of roaming into the enemy jungle for chimes and be sure to use wards and oracle lens to avoid getting caught out. You can also use your W to check bushes, as it gives you vision for a second after you place it. Most of the time you shouldn't grab chimes in the enemy jungle unless you have teammates with you, you know where the enemy jungler is, or you have their jungle warded. This applies less in the late game if you have taken towers, but still be cautious whenever heading into their jungle without intel.

Traveler's Call (passive)- Bard's passive is what makes him inherently a roamer, as it spawns chimes around the map for him to collect that grant him some temporary move speed and mana, and grant him a chime stack. Every 5 stacks, Bard empowers his meeps to do an additional 12 magic damage, and occasionally reduces your meep spawn cooldown(down to 4 seconds), or increases your maximum chimes(up to 9). Additionally, at 5 stacks, meeps now slow from 25-75% for 1 second. At 15, meeps hit around their target as well as behind in a cone. At 35, the cone's radius increases.
Meeps (passive) - Every 8-4 seconds(based on chime stacks) a meep spawns that is consumed on an auto attack, dealing additional magic damage. Meeps count as separate spells from the auto attacks, so you can proc something like electrocute with one auto and a Q. This is also super useful for getting your support item completed because each auto attack with a meep procs it twice.

Cosmic Binding (Q)- Bard fires a projectile that deals magic damage and slows by 60%. After hitting an enemy, it continues for 300 units. If the continuation hits another enemy or a wall, it stuns both targets. This is Bard's only offensive ability and it can be incredibly powerful, as it can stun 2 people for up to 2 seconds, or just slow a target. You can also stun someone with this through a minion, though they have to be pretty close to the minion. Once you have 5 stacks, you can make this much easier to hit by first autoing someone for the meep slow. This ability also works with any terrain created by champions such as Azir's or Anivia's walls.

Caretaker's Shrine (W) - Bard places down a shrine that heals and grants movement speed to any ally that walks over it. Over 10 seconds, the shrine's healing becomes stronger, up to around double of the minimum. Bard can have up to 3 of these at once, and can cast them directly on an ally, which doesn't count as one of his 3. It also grants vision around it for 2 seconds, so you can use it to check bushes. If an enemy steps on a shrine, it becomes destroyed. Shrines never expire. These can be useful as a general heal and movement speed boost, and in lane you should place them along an escape route, and make sure to place them at a small distance to take advantage of the movement speed. Also make sure to not make it easy for the enemy to just walk up and destroy your shrines. You can also leave a shrine in other lanes whenever you roam just to help them out a bit.

There is also in interesting mechanic that you can use with self cast that allows you to put a shrine on the floor as well as yourself at the same time. This is explained further in the next chapter.

Magical Journey (E) - Bard creates a one way portal through terrain that lasts for 10 seconds. Anything that goes through becomes revealed during travel, and allies get a 33% speed boost while traveling through the portal. You can flash out of it and be hit by abilities while going through it, so you can potentially get grabbed out with something like Blitz Q. This is bard's most versatile ability, as it can be used for escape, engage, flanking, ganking, distraction, speed, and much more. If somebody is chasing you through a portal, you get to Q them for free, as they will end up against the wall when they exist the portal, so unless the enemy champion has super good chase, you can get away almost every time as long as you don't get stunned.

Tempered Fate (R) - Bard throws magical energy to a target area and puts everything(and I mean everything) into stasis for 2.5 seconds. This has a pretty decent range, but it is thrown in an arc, so the further away you are from the area, the longer it will take to get there. It also lights up the ground underneath before it hits, so your enemies can dodge it. It can also put epic monsters or towers into stasis, which can be useful for steals or tower dives. This ability is mainly used for 2 purposes. Allowing your team to set up, or denying certain people from participating in a teamfight for a few seconds. If you hit 2 people with it, or a target frozen is near a wall, you can get an easy Q on them for your teammates to take advantage of. This ability decides teamfights, so being able to isolate carries and ulting them can change the course of a game. It can also set up other champion's abilities super well, such as Neeko's ult.

If you self cast W and normally cast it on the floor at the same time, it will do both actions in quick succession. It also does not work if the normal cast is on an ally. It helps if you bind self cast to a separate key, as it is a little difficult to be able to press Alt + W and W within the time you're given, although it is possible. I've bound it to Z so I can just press Z and W at the same time and boom, double shrine. (It still uses the same mana for 2 casts)

Now for the meat of the guide. Every summoner can be useful depending on the situation, so knowing how to use them is incredibly important for this build to succeed. Firstly, some general rules for Unsealed Spellbook. Spellbook has a 300 second cooldown, and an additional 60 seconds at the start of the game, so your first swap is at 6 minutes. Each time you select a unique summoner spell, it decreases the cooldown by 25 seconds, up to 6 times(150 seconds). After you swap to a different summoner, it announces this fact to everyone in the game in the chat. You also can't use a new summoner for 5 seconds after you swap to it(excluding smite), and you have to be outside of combat for 5 seconds to swap, so you can't just pop a new spell in the middle of a teamfight. There is also a 10 second cooldown after you use a swapped spell to cast your original spell. You have to swap 3 more times in order to choose the same spell again. In general, until you get all 6 stacks, you should swap spells whenever possible to get the cooldown low as fast as possible.

Barrier - Barrier is pretty self explanatory. It is really good early game if you need to peel for your adc, or just find that the opponent hits pretty hard. This becomes a lot less effective late game as you will have Locket of the Iron Solari already, but if you need the survivability, then it can definitely still be viable.

Cleanse - Only take this into a heavy CC matchup. Apart from that this is a pretty useless summ early game, and most of the time even in a CC matchup something like barrier or heal is probably better early game. Late game however, this can be super helpful in avoiding getting bursted down if you don't have Mikael's Blessing.

Ignite - You should probably already have this summ, but it is good to keep in mind that most of the time you want to take up your flash slot for swapping unless this is already on cooldown or you really need survivability. This summ is really useful for applying grievous wounds, and it can also be used with exhaust to hyper focus somebody.

Exhaust - This summ is SUPER useful. It slows, as well as reduces damage output, so it is really good for focusing down a carry or just disabling them in a teamfight. This is especially true if you also ignite them. It is also helpful if you are trying to escape from someone because of both of the debuffs. In lane, you want to use this on the adc and focus them down, as they provide most of the damage and typically don't have great mobility so it should be easy to pin them down with the slow.

Flash - This is the main slot that you will be swapping spells with, as you already have a lot of escape with your kit, and it is better to have 2 summs that impact the fight rather than one. The only reason why this wouldn't apply is either if you are getting absolutely destroyed by an assassin and need 2 defensive summs like flash and barrier, or your ignite is on cooldown.

Ghost - Ghost is actually very good on Bard, as it is very versatile. The first and obvious reason is if you just need the movement speed, whether you are running or chasing. Ghost also helps you position yourself for a Q stun, but this can be strange to swap to if you don't know if a fight will happen soon. If there is nothing else you really need, and exhaust is down, then this is a good summ to hold, both offensively and defensively.

Heal - If you need to keep you or your adc alive, then heal is super useful for this. It also grant a short speed boost, whether you are chasing or running. If you don't have a specific spell you want to swap to, but you or someone needs healing while out of combat you can quickly swap to this and pop it. Most of the time however, this summ is only really helpful early in lane because late game your W heals plenty.

Teleport - If you ever see an opportunity to teleport, just go for it. This summ helps Bard roam even more, and makes top lane much more accessible early game, so make sure to pay attention to every lane. You also might want to set up wards in good tp positions, especially if someone else on your team is running tp.

Smite - This is the only summoner that doesn't have a 5 seconds cooldown after swapping to it. After 2 swaps, this summ gets upgraded to do 900 damage instead of 450, so it can be super useful after your first 2 swaps for stealing objectives, or just helping secure them. It can also give you a quick heal if you're in the jungle, by just using it on a monster.

Inspiration:

Unsealed Spellbook - I really hope you read the prior chapter

Perfect Timing - This helps you get a stopwatch and saves 650 gold on your Zhonya's Hourglass. Don't hesitate to use it early to prevent your death or bait for your teammates.

Future's Market - This helps you rush your items a little bit quicker and gives you a tiny gold advantage early game, and can possibly help you sustain a lead mid game. Don't waste it on smaller items though because it costs 50 gold every time you go into debt, so you should only really use it for full items, and epic items depending on how much of a spike it will give you. You can also take Biscuit Delivery if you don't want this, but bard has really good mana sustain with his chimes, so he doesn't really need this and it doesn't help late game at all.

Cosmic Insight - Gives you cooldown reduction on both items and summoner spells, both of which are really nice to have with this build. The other options just don't really do much for Bard compared to this rune.

Domination:

Zombie Ward - Spawns a zombie ward each time you destroy an enemy ward, and grants you adaptive force each time you do. All 3 of the runes in this line are good, and while Eyeball Collection will probably get you max faster, Zombie Ward helps you throughout the game, and gives you more vision. You can also take Ghost Poro if you have a habit of warding a lot in the enemy jungle, as it will extend the amount of time that you have vision there.

Ingenious Hunter - With 3+ active items, and the insane new buff, this one is a no brainer, and while you could also take Relentless Hunter, the impact of Ingenious Hunter is just much better with this build.

Locket of the Iron Solari - Grants a shield to all allies around you. This item is super useful for blocking aoe damage or just saving one of your allies. Most of the time in a teamfight, if an ally drops to half health, just pop this and try to get your team to engage. It isn't worth trying to get a clutch shield on somebody with the chance that they die, your shield will block damage no matter when it is used, so try and use it whenever it will block the most potential damage

Zhonya's Hourglass - This grants you a lot of survivability and the opportunity to engage and try to get a Q for your team, and then Zhonya's to prevent your death, and give your team an opportunity to dive whoever you stunned. It also just helps you stall out cooldowns or just avoid a big ability. Don't ever hesitate to use this if you think you have to and it will benefit you in any way.

Redemption - This item lets you target a large area and, after a few seconds, heal all allies and damage all enemies within it. You can also cast this from death. If you see both teams grouped up in a concentrated area, go ahead and pop it in the middle of them. This can also be set up with your ult, if you hit a couple enemies with Tempered Fate, then put redemption down on the same area, your team can group there to restore health, while also dealing 10% true damage to all the enemies you froze.

Abyssal Mask - Gives you some health and magic resist, as well as giving anyone you immobilize 15% increased damage for 5 seconds, this also works with your ultimate. Just a good late game item for helping your team shred through anybody you stun.

Mikael's Blessing - If your carry is getting constantly stunned, you might want to build this just to help keep them alive. It removes any cc on an ally while also healing them. Also gives you some magic resistance. This item may be hard to use on somebody else though, as most CC don't last too long, and it may be difficult to pay attention to somebody else getting stunned while still being useful yourself, so you may want to practice with this item before taking it into ranked.