Polymer clay is an outstanding medium for making papercasting molds. It keeps fine detail, doesn't change at all when baked, and releases the paper cleanly without needing mold release agents which are usually non-archival. Sculpey Premo has proved to be the best choice for this technique, due to its strength and flexibility. It is by far the most economical in 1 lb. bricks.
You will also need a roll of waxed paper on hand.
White is a good choice as your design will show up well and it is easy to tint with small bits of other colors when desired.
First of all, the clay must be "conditioned" as the various chemicals separate over time, and to make it more malleable. The easiest way is to break or cut it into chunks (no bigger than the little 2 oz packs) and hand them to your sons (who are watching a football game) to knead. The next easiest method is to warm and flatten in your hands until it will go through a pasta machine at the widest setting.
*You should not use a pasta machine for food if you use it for polymer clay.
Keep folding but never roll with the fold last as you can add air bubbles. Every time you roll you need to be conscious of avoiding getting air trapped between the layers - usually folding in thirds to enclose rough edges and rolling with the folds perpendicular to the rollers will work well. If you think you have a bubble, prick it with a pin. Even when your mold is almost done you can still do this and gently smooth over the pinhole.
When it is nice and workable you can start dialing down to the thickness you want. Make a habit of always having waxed paper under your rolled clay.
If you are making a mold with several layers you want to work the clay quite thin - usually the thinnest setting doesn't roll smoothly or hold together well enough, but about the second or third will do fine. For background layers or very bold designs you can stick with the thickest setting. It may take some experience with cutting to know for sure - it seems thicker when cutting than it did when rolling. Start with very clean and simple designs (*upcoming tutorial on choosing and adapting designs).
Next time: joining strips of clay to get a piece large enough for your casting design.
remember, live classes are available - go through the entire process; first a day of small projects to learn several techniques and see the results, then discuss how to plan successful casting projects using your own art and a day to do a big project with plenty of individual attention, classes that meet a third day (or half) get to see the test castings, learn how to fix problems in the mold, work with color, learn about 3-d possibilities, and more) - click here for information on workshops