The core items are the wriggle's lantern then the sheen because everytime u do an attack, your next basic attack deals double damage equal to your basic attack so its really good since lee sin relies a lot on his abilities. With those i tems, you should be able to make enough kills to be a little bit OP. then comes the tanky Lee. Now everything depends on the game. If major opponent on the other side is strong in the AD (attack damage) or AP (ability power) then you must go into a thornmail or force of nature if the situation really requires it. if not, if u can handle well the other team, go straight into warmog so you get more life tto upgrade your atma's impaler passive, converting 2 % of your life into AD. then you sell your wriggles to have now a blood thirster and boost zeal as your 6th item to get attack speed and movement speed to help your lifesteal. and if your build is complete at the end and you still have money, buy potions that give you extra cooldowns, life, AD and invisibility vision.