The easiest way is to buy one, undo the quick release, lift the seat pot off the bike, lift the quick release off and put the allen head on instead ( make sure the gaps line up) , then slide in the post and tighten the allen key.
If you want to save a few $ then take an allen key tool to the nearest university....and swap your quick release for the paranoid uni students allen key clamp. You'll be doing them a huge favour, they'll freak out and save a fortune by stopping whatever drugs they used last night.