This little instructional illustration says it all.
If you’re into little papercraft characters, you must head on over to Cubeecraft and grab these two cut-out kits of Half-Life’s Gordon Freeman, and his perfectly fitted headcrab accessory.
(Note: Cubeecraft appears to have temporarily succumbed to a bad case of headcrabs itself, and is currently down. Be sure to bookmark the links above if you want to print out these cool models later. For now check out low-res screens of the templates at DeviantArt here and here.)