Look for an early gank at mid lane or bot lane, every damage you do to enemy laner is good, ganks at bot lane without R are dangerous, because most of the time you end up stunned and dead, so be careful when ganking bot lane at early level. You get the the items that every Lee Sin needs and then the other 4 items depend on the enemy team. The Tiamat who you rush (if you can) needs to be left for the last item, buy every other item and left it for the end, at the end you eiter build Titanic Hydra or Ravenous Hydra.

You need to build the Titanic Hydra if you don't have enough health and enemy team does a lot of damage or if your team needs a little bit of more tank. You need to buy the Ravenous Hydra when you build your Lee Sin for ONLY DAMAGE! Ravenous Hydra gives you a lot life steal and damage, so be careful!