STEP ONE: Switch on your wax pot! We've got a specially designed (if somewhat ancient) wax pot in the studio. You can use also use a double boiler over a hob. Two suggestions for safety: never melt wax over direct heat, and never leave your wax pot unattended.
STEP TWO: You need a plaster mould. It MUST be what I'd call a "drop out" mould - one where you can turn the mould over and your material will...drop out. Something without undercuts, and that doesn't get narrower towards the top.
STEP THREE: The mould needs to be saturated with water. Drop it in a bucket of water, and leave it until bubbles stop appearing. (If you wander away and forget, don't worry - but don't leave it for hours and hours, as the plaster will eventually start to deteriorate). The mould at the bottom of this picture has just been dropped in the bucket - and you can see some big bubbles.