The idea of the build is to increase attack speed and critical's, this makes for huge damage and make you almost un-killable because of the bloodthirster.
A great move for lee to do is flash ahead of an enemy, then ult kick them back to the team. Aim for picking people off because you can probably 1v1 anyone but a fed tryndamere with his ult. Because Lee is new people are still focusing him so try your best to not feed and jungle in down time.