You will need
- 2 iron ingots (smelt iron ore in a furnace)
Then craft a shear from these two iron ingots, at a crafting table. Next, approach a sheep(with shears in your hand) and press "shear" .
Note: It takes a while for achievements to pop in this game. It seems as if every 2 minutes or so all achievements will unlock, which you've unlocked in the mean time.