Look up lee sin combos on youtube. such as
Q/R/Q
R/Flash
Q/Q/Ward behind enemy/Ward hop behind/R
Flash/R
Ward max Range placement/Ward Hop (try to do it as fast as humanly possible, it's actually something that's easy to fuck up)
Look up all those combos from youtube, then go into practice tool and literally just spend hours practicing them. I have to say Practice tool is the greatest gift to lee sin players. Lee Sin has SOOOO many combos that are impossible to practice in a normal game. So use practice tool instead to practice them before you even do a game on lee sin with them.
Also practice on how to use ur passive properly on camps.