Step 1: Materials neeeded
Pottery plaster. It HAS to be ceramic grade, otherwise it will dissolve in water very easily. (please wear a NIOSH mask when working with dry plaster. Avoidance of lung cancer is so very worth it...),
A non-important bucket,
The objects you want to make the mold from (must be flexible plastic, unless you want to cut it up to get your mold out),
A spray bottle of vinegar (not pictured),
Something to stir your plaster with.
Step 2: Mix your plaster and pour
This stuff sets pretty quickly, so use your stick to stir stir stir and get all the lumps off the bottom as quickly as you can and then pour it into the molds.
Give the mold a couple of good taps (not hard enough to spill the plaster) on a sturdy surface to get any bubbles out from the bottom.
I used 1.5 lbs of plaster and was able to make one good sized mold and one rather shallow one.