Use plaster of paris, pin down the worms you want to pour.Then make some sides using some strips of thin wood about 1/4 " thick by 1" tall. Pin the worm on a piece of cardboard covered with Saran wrap. Spray with Pam. mix plaster and place the wood around the worms( i used 12 worms) ,making a square box then pour plaster and let it set up. When plaster is dry use a small paint brush and paint the cavity and top of the mold with Elmers glue. The worms will not stick in the mould.. I have poured thousands of worms over the years using this set up.

Depends on what you want to mold and whether one or two-part mold.
I wanted to copy some jig trailers and figured plaster of Paris the fastest, easiest and cheapest way for a one-flat-sided object.
Uncle Josh had gone out of business and their pork frog which I was running out of was one of the best. Here's the molds:

I can't remember the coating off hand used for plaster, but it is necessary.

The other is a craw trailer that has a nice flapping action on the swim or drop that required a two-sided mold.
For the two-sided plaster mold needed for round lures, I did one side, let the mold dry, coated it with a sealer and then smeared vaseline on the surface. With the lure in the cavity, I poured the next side of plaster on top; let it dry and coated the surface with sealer. I used a Dremel to make a pour hole at one end of the lure. End result:


Now I don't bother. Molds are cheap on Amazon and eBay and aluminum to boot for great detail.

YouTube Worlds Worst Fishing. Lots of good info on his channel