Urethane Resin - Liquid Plastic
Molding Compound - Quick-Sil
Disposable Measuring containers
Step 1: Create the Original
Step 2: Quick-Sil Mold
Next, just push it onto the hardened object making sure to fill in all the details. It's a little harder than play dough and pretty malleable. Also, attempt to get the back as level as possible since that will be on the table when you cast.
I like Quick-Sil since it's reasonably priced if you're using it for small objects. The 3oz set should be under $10. Anything larger than this symbol and you'd be better off using a different silicon or latex mold material. Both of those are a whole other animal that I won't get into, but I'm assuming someone else on this site has done a fantastic job covering.