I mained lee sin a lot past couple seasons. Havn't played league in a while.
Basics. Start off with cripple(E) lvl 1. Press E, 2 auto attacks then E again then 2 auto attacks.
Basicly you take advantage of his faster attack speed with only 2 auto attacks before the passive goes away.
For example. When you are going into a team fight.. (W) to a minion/ally/ward close to an enemy player then maybe lay 1 to 2 auto attacks. Then press E twice to slow them down. Lay 2 more auto attacks because of your passive. Then since you slowed them down and should be right next to your enemy by now. You will throw your Q because it will be very hard to miss, then lay 2 auto attacks. After that your opponent should be very low, press Q to connect your sonic wave (Since it has 8% missing health as attack damage). It should almost garantee you the kill and you took advantage of your passive the whole fight.
Tldr: Lee sin passive last 2 auto attacks. Cast a spell then 2 auto attack then cast a spell
Tip: If you are low on health press your (W) twice and cast smite, you life steal from smite.