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.
Step 3: Remove bubbles
Step 4: Wait, wait, wait and then remove the mold
Gently tap and wiggle the mold out of the plastic.
When it is out, you can lightly and very very gently sand or rub out any imperfections. But as long as you are not pressing your clay on very hard, you probably won't pick up the divots that may be left in the plaster.