easy peasy.....I use amaco clay....no 25. I buy it in 50 pound boxes for 22.00 I can get boo coos of molds out of the box for both glass and clay.

first I sculpt the piece I want the design in. let it air dry. to what is called bone dry. put on the separator which keeps the clay from sticking to the clay when you try to cast the item you just sculpted.  and then take a kneed amount of clay and push around my piece. now if I want a perfect size then I get empty juice box and cut the top off to where it stands about 3 inches deep. then put my item in the middle and put the clay on top and press till all of the edges are filled.flip it over and carefully release the piece from the box. I fire it in my kiln on the flowing stages

take 1 1/2 hours to get to 200 degrees

hold for 60 minutes

then 300 to 400 degrees per hour to get to 1945 degrees

hold for 10 minutes

go down 3 hours to get to room temperature...

your piece you carved will pop right out and you will have a great mold. you can sand where you need any excess edges.  

your ready to make your polymer or glass molds now.