Loop your yarn like this – so the tail end (the end that is cut) is behind the working yarn (the end attached to the yarn ball).
Pinch the section where they cross over.
Reach through the circle (front through to back) to pull the working yarn through the hole.
Pull the tail end so that the loose knot tightens.
There you go!
Put it onto your hook and pull the working yarn to tighten.
You use a slip knot to start almost every amigurumi piece, before you know it you’ll be able to do this in your sleep.