For first i want to excuse grammar or other errorse because i am not English :)
In the early game it is highly suggested to do the blue buff,gaining a notable advantage on mana regen.
Essentially you must spam alpha strike on enemy taking advantage of its range,damage and no attack engage risk.
it's also important to kill much minions , for the gold you need to the items that can make you win,and concentrate on doing early aggressive kills on enemy champions letting yi to be fed and raise his damage power.
Ganks from the junglers are important in early game because this is a carry build and obviously kills let your work with Yi much easier.
The most important items in this phase are (for me) Berserker Greaves and the zeal ,those can make you deal consistent damage in early game,using alpha strike and wuju style.

This phase starts with the buying of Stattik Shiv, with the enormous help this items gives to you in 1vs1 you must concentrate on having kills and ganks with highlanders,to exploit every single kill occasion.
Proceeding in game if you use your occasions in best of ways you will already be at 4 kill minimum and you will be able to kill everyone in 1 vs 1 if not a fed ap in mid or a fully armor built tank.

This is the best part of playing yi, you will have stattik and phantom dancer, so now you have to decide if buying health items (Giant belt,then warmog) or increase your critical strike damage with Trinity edge,if you build healthy it is because you re having trouble in the match with damage, otherwise,if you're doing well you have to buy trinity edge,because its combo with critical is purely beast.
In this game phase you must concentrate on win team fights,1vs1,2vs2 and all the fights you take part to.
If u are in a difficult situation on tower situation,your attack speed is very useful,and if you combine it with a desperate highlander you can take down a tower in matter of seconds.
One thing i forgot is the boot speed enchant, you will have already 430 mov spped without it , but if you are like :"I wanna have 600 mov speed with highlander" you can buy them,it s a nice combo.