First, make sure all the materials are at room temperature. Cold products are thicker and slower than indicated on their Technical Bulletin.
For two-component systems, the accurate measuring or weighing of Part A and Part B (or Resin and Hardener) is essential. For mixes less than 8 lb, you should use a gram scale that reads to 1/10th of a gram. Polytek sells an inexpensive gram scale with a capacity of 5,000 grams, or about 10 lb. For larger mixes, you can use a pound scale that reads to 1/10th of a pound. Following this guideline will allow you to measure your product more accurately.
Start with a clean mixing container that holds about twice the amount of liquid you will mixing. This allows for enough head space to thoroughly mix the rubber (a five gallon bucket holds 40 lb of rubber, so avoid mixing much more than 20 lb of rubber at a time).
You can use a tongue depressor for small mixes in cups, but use a Poly Paddle or similar mixing device whenever the mixing container is large enough to handle it.
Pour the correct amount of Part B into the mixing container first, then add the correct amount of Part A. Begin stirring the mix in a figure 8. Stirring in a circle will only spin the material rather than mix it. Scrape the sides and bottom of the mixing container as you mix. Stir until you have a uniform color and viscosity. Some people like to use a drill mixer. This is fine but only mix on the slowest speed to avoid adding air to the mixture. You will also still need to scrape the sides and bottom of the container by hand to ensure thorough mixing.
Once the rubber is mixed you can pour it into your mold box. Only pour whatever comes out of the container. NEVER SCRAPE THE INSIDE OF THE MIXING CONTAINER WHEN POURING YOUR MOLD. It is tempting to try and get every last drop of rubber out of the container, but you will likely pull out some unmixed Part A or Part B and end up with a sticky spot on your mold.
To minimize the chance of getting unmixed material in your mold, some people use the 2-bucket method. One bucket is used for mixing the rubber, the other bucket is used only for pouring the rubber. After the rubber is mixed in your first bucket, pour the mixture into a clean second bucket. Then pour the mold from the second bucket. This way your pour bucket never has just A or B in it, only mixed material. This is especially important for larger molds requiring multiple pours to complete.
When you are done, clean your paddle with denatured alcohol (Denatured alcohol is a good cleaning solvent, but must be handled with extreme caution owing to its flammability and health hazards). Leave the excess mixed rubber in the buckets - it will cure overnight and peel out the next day. Then you can use denatured alcohol to clean out any residue left behind. Note that the residue is unmixed material that could have ended up in your mold if you had scraped the pail.