starting whit boots for defense, harass continuosly your enemy,untill he have 1/3 life, then Q+autoattack+ignite and the enemy surely die,( mushrooms are preetty usefull )

ocasionally item like : mallet and rod of ages could be exchanged for the zonyas hourglass,thornmail or raduins shield....
if you want more offensive you need lich bane...

must have item: sunfire/force of the nature/ninja tabi: whit theese items and the previous mastery/runes/spells you become very fast and tanky.
is extraordinary the harass of that build,but the really strong is in teamfight 5 vs 5.....
counter for this teemo: malzahar,kassadin, anivia ( required quicksilver sash and banshee veil for counter them )