Hey, my LoL name is StroopWafeI, and I play master yi and ashe a lot. These 2 champions are the main reason i climbed from silver 3 to plat 3 in 2 weeks. Master yi is an underrated champion, his win rate is not that amazing at the moment because most people don't play/build him right. So why not make a guide?

The runes i currently use are the following:

9x Greater Mark of Attack Damage +0.95 attack damage
9x Greater Glyph of Scaling Magic Resist +0.16 magic resist per level (+3 at champion level 18)
9x Greater Seal of Armor +1 armor
3x Greater Quintessence of Attack Speed +4.5% attack speed

These are not too expensive and help you out a lot. You dont really need the early magic resist for your clears, so scaling MR is a lot better. At level 8, you already have the mr that normal mr runes would give you.
The armor helps you to not take too much damage till you finally buy some defensive items.
The AS quints are amazing with your passive, your q and your e on hit.
The AD marks help you with your early clears.

I think there is a slightly better rune page for Master Yi than this one, but it will cost a lot more and personally I'd rather spend the IP on new champions and stuff. Anyway, here is the optimal rune page for Yi:

9x Greater Mark of Attack Speed +1.7% attack speed
9x Greater Glyph of Scaling Magic Resist +0.16 magic resist per level (+3 at champion level 18)
9x Greater Seal of Scaling Armor +0.16 armor per level (+3 at champion level 18)
3x Greater Quintessence of Attack Speed +4.5% attack speed

The marks of AD are switched for AS, because this is probably the best offensive stat on Yi, getting the best out of his passive, q and e.
The armor seals are switched for scaling armor seals, these will help you with the late game. If you are not planning on ganking early, which is totally fine, these seals are great.

So in short:
Always take AS quints
Take 1. AS marks or 2. AD marks (does not matter too much)
Take 1. MR/lvl glyphs or 2. flat MR glyphs (MR/lvl is a lot better imo)
Take 1. Armor/lvl seals or 2. Armor seals (Armor/lvl is better)

Fury > Sorcery
AS is a great stat on Yi while AP only scales with his w

DES > Feast and Expose weakness
To do as much damage as possible

Vampirism > Natural Talent
I'd say you could go both of those, you can take the one you like.

Bounty Hunter > Oppressor
You will probably kill everyone on the opposing team at least once, so 5% bonus damage is nice. You also don't have any cc yousrselve, so you need teammates with cc to use oppressor.

Battering Blows > Piercing thoughts
You are ad.

Fervor is the best keystone for any AS champion, so ALWAYS take it.
Savagery > Wanderer
Nice little help at clearing in the begin.

Runic affinity, because the extra buff time helps your clears and ganks. You wont buy normal potions, and you won't fight too many people 1v1.

Merciless > Meditation
5% bonus damage is A LOT, and you will not be oom (out of mana) too often.

Dangerous game > bandit
Bandit is for supports or gangplank, dangerous game is great for dives and other close skirmishes.

On Yi, you should really only build the items that are in this list, and even then some of them are really situational.

Best offensive items

Bloodrazor
The best jungle item for Yi

BORK
The only item in the game that offers AD, AS and lifesteal. All great stats on yi.

Youmuu's Ghostblade
Gives flat armor pen, which is great against squishies. Also gives you great chase or flee potential, plus AD and AS.

Sterak's Gage
Makes you extremely hard to kill when combined with bork and some resistances. The better version of hydra for Yi.

Ok offensive items

Titanic hydra
Gives some hp, ad and waveclear. But there are better options, since we dont really need the waveclear.

Maw of Malmortius
The damage, mr and passive are ok.. But it makes u a bit weak against AD champs, since it doesn't give any armor or health. I would only recommend buying this against super ap heavy teams.

Guinsoo's Rageblade
If you want to go on hit.

Phantom Dancer
I don't really like only 1 crit item, but some really good Yi players build it so it must have potential.

Best defensive items

Dead Man's Plate
You should build DMP almost every game, the stats as well as the passive are amazing on Yi.

Randuin's Omen
Reaaaally good when the opponent has multiple critters, or a really fed critter. Helps you stick to your targets as well.

Banshee's Veil
Banshees is amazing on Yi, it gives us a bit more time to deal our damage. The banshees passive can, for example, prevent a blitzcrank from hooking and stopping us. This item can really win you a teamfight.

Ok defensive items

Spirit Visage
The slightly worse version of banshees imo. We dont really need the CDR, cause if we do not get resets in teamfights we probably lose/have lost them anyway. It also doesn't provide the amazing banshees passive.

Guardian Angel
Can be really good if the game is going very late.

Thornmail
Can be amazing against multiple auto-attack based champions, or when the opponent is full AD.

Sunfire Cape
The item is ok, but I think the passives of Randuins and DMP are slightly better on Yi.

Pros

Has a nice first clear
Has build in sustain
Scales like a beast
Does tons of damage
Hard to kill
Can dodge A LOT with his Q

Cons

Squishy before you have build some resistances
Early ganks are not great
Taunts and stuns

When going jungle Yi, smite is an absolute must. But there are multiple spells that you can take for your 2nd slot.

1. Flash
Flash enables you to get over walls, have a safe escape and set up big plays. For me, it is the best summoner spell on Yi after smite.

2. Ghost
Since ghost got buffed some time ago, it is actually a pretty viable spell on some champions. Some champions were also indirectly buffed because of the ghost buffs (like heca, olaf). Even although it is on a very short cooldown, I still think ghost is not as good as flash on Yi. Your ultimate and youmuus will help you be very fast anyway, and you can also not be slowed during that duration.

3. Teleport
Meh, not the best spell on Yi, but some people like to use it for teleport ganks or super fast clears. Personally, I don't really like it, but it is used sometimes...

As Yi, your goal should almost always be hitting lvl 6 ASAP. Considering your early ganks are not great, you will have to find another way to hit 6 fast than ganking. What I always do is the following route:

Red side
Start with machete and refillable potion

Start at gromp
AA the gromp once, then use q immediatly. Smite gromp and AA till it's dead.

Go to wolves
AA the big wolf, use q immediatly, right before your passive will give you a double AA, use your e. Kill wolves.

Go to raptors
AA a small raptor, use q immediatly. 2 small raptors will be alive as well, aa them both once. Then activate e, and kill the big raptor.

Go to golems
AA the big golem once, use q immediatly. Smite the golems, aa till theyre dead.

Then B and do a full clear.

Blue side

Go to golems
AA the big golem once, use q immediatly. Smite the golems, aa till theyre dead.

Go to raptors
AA a small raptor, use q immediatly. 2 small raptors will be alive as well, aa them both once. Then activate e, and kill the big raptor.

Go to wolves
AA the big wolf, use q immediatly, right before your passive will give you a double AA, use your e. Kill wolves.

Go to gromp
AA the gromp once, then use q immediatly. Smite gromp and AA till it's dead.

Then B and do a full clear.