Flash and Heal has been the standard ADC summoner spell combination since possibly mid-season 3. In fact, Flash has been the standard summoner spell to take on almost every single champion since forever. It's always been indisputably the best summoner spell in the game, so there's nothing you can replace it with.

Heal on the other hand, replaced Barrier when it got buffed in season 3 to give movement speed on the cast, which is the reason it's been standard over Barrier ever since, despite being affected by Ignite's and Grievous Wounds. Aside from the small movement speed boost, which can be used both offensively and defensively, another reason why it's stronger than Barrier is it affects an ally, which is perfect for bot lane where you're laning with a support.

The reason you want a defensive summoner on ADC is that you have a lot of DPS, so the longer you can survive, the more damage you'll put out, usually out-damaging what you'd get from Ignite. And if it can go as far as to save your life, then that's even better, as ADC's are very important to have alive for both providing DPS and taking down objectives.

Bloodthirster: Is a item that gives you more life steal then blade of the ruin king so its great to have for just life steal and the damage it gives you and its one of the items for you to get when you need more substain or when you sell your guardian angel cause its down

Mercurial Scimitar: This is a great item when fighting against a team with lots of cc and it's a really good item if you have great reflexes to get away from the cc and flash away to make the great escape this item can replace your hurricane or lord dominiks depending on your situation

Mortal Reminder: A great item that inflicts grievous wounds (Grievous Wounds is used to limit or minimize healing of enemy champions so as the summoner spells or Soraka) and this can replace your lord dominiks

Maw of Malmortius: When fighting against really heavy ap like 3 people this will help and save you a lot of time that is a promise I can make and sometime you can just build this for 1 person like Katarina ap carry or just any ap carry for that matter

Blade Of The Ruin King: Another item to replace your Guardian Angel when its down cause you sold it, it basically like the Bloodthirster accept getting when fighting a lot of tanks you should get this or I at least recommend it[/color

Guardian Angel: This item can be very useful for when fighting against a bunch of divers such as Talon or also helpful if you end up in a bunch of 1v1's this can replace your hurricane[/color/]

Clean Cuts: After casting an ability, Xayah's next 3 auto-attacks will strike all enemies in a path and deal 50% damage to all secondary targets. At the end of their trajectory the feathers will remain on the ground, ready to be pulled in by Bladecaller. The feathers themselves are hard to aim in lane and do very little damage anyway when they do hit, but the fact that the feathers do so much damage and bring so much utility with Bladecaller is the most important thing about this passive.

Additionally, Xayah has an additional passive when paired with Rakan, which allows her to join in on Rakan's Recall to get to base faster.

Double Daggers: This spell is decent harass, but nothing special. Its main use is getting 2 feathers behind your enemy so after one auto-attack you can cast Bladecaller and get an instant snare. Or if you're aiming for DPS, to get a total of 5 feathers behind your enemies including Clean Cuts.

On release you could auto-attack during her Q animation, but in patch 7.12 it was removed. However, in patch 7.13 they made her Q auto-attack lock duration scale with attack speed, so once you have enough attack speed you'll be able to auto-attack during your Q. This is useful for making Double Daggers basically throw out 3 feathers for an instant snare, and so that casting Double Daggers doesn't reduce your DPS late-game but leaving you unable to auto-attack temporarily.

Deadly Plumage: This spell is actually a really strong auto-attack steroid, and Xayah's DPS is massive while it's active. Comparing it to Ashe's Ranger's Focus for example, it gives 10% more AS and increases her AA damage by 7.5% more. The extra AS also helps Xayah get out feathers faster to burst people down with Bladecaller more swiftly, making Xayah very deadly in duels while Deadly Plumage is active.

Bladecaller: This spell does both massive damage depending on how many feathers you have on the battlefield, as well as brings a lot of utility in the form of spammable snares. With 30% CDR all your spells will have low cooldowns to constantly generate feathers, and Bladecaller will be on a low enough cooldown to use it every time you have enough feathers out.

Depending on the situation you can either activate this immediately after getting 3 feathers for an instant snare, or you can take your time to get as many feathers out as possible before casting Bladecaller either when you want the damage, or when the enemy starts to retreat and you want to stop them in their tracks.

Featherstorm: Between this ult and Bladecaller, Xayah is a nightmare for divers and assassins to deal with. The uses of this spell range from becoming untargetable while you're getting dived, dodging skillshots, damage, and quickly getting enough feathers for a snare.

If you're wanting to snare someone with the feathers generated by Featherstorm, you have to be sure that you're really close to the enemy you're trying to snare, since a max range Featherstorm followed by Bladecaller will only hit them with 1-2 feathers, compared to the 3 needed to snare someone.

Pros
. Xayah has lots of CC for a adc
. Xayah/Rakan can back really fast with her passive (Only when paired with Rakan)
. Xayah is amazing in team fights with her ult used right to root the enemy team
. Xayah can burt through towers with her w
. Xayah never really falls off and can come back almost anytime in the game
. Xayah over all just looks cool and is fun to play

Cons
. Xayah is super squishy (Like all adcs but still thought I should mention)
. Xayah can be kinda of a hard champ to master
. Xayah This is mostly my opinion but I think she hard to farm with
. Xayah has a bad boyfriend!

In conclusion these are the big things I watch out for.