In art and design school this past fall, I did some experimenting with dental-grade alginate, clay, plaster, and color additives. This is not a step-by-step I-ble. I just wanted to share pics of three separate plaster casts, all of my hand. The white hand w/ my mouth was an actual assignment. The other two hands were just for fun, experimenting with fabric dye and concrete coloring mixed in the plaster. One of the hands came in "handy" for a wood carving I did that semester.
If you can't get alginate and plaster through school, it is available on amazon or many suppliers. The entire process requires two mixes: 1)alginate/water to make the negative mold of your hand or chosen object, 2)plaster/water to make the replica of your hand from the alginate mold (coloring added to the white plaster optional).
The mixing ratios for the alginate and plaster varies from manufacturers, but it is important to follow the recommended mix ratios using an accurate weighing scale. It may take trial and error. Expect to make a few mistakes, part of learning. The finished results are impressive.