I use a mortar and pestle and use the method used in the capping direction sticky:
"Now that we know how much active compound and filler to use (Aspirin and Baking Soda) we need to mix them together. To do this we'll use a mortar and pestle with the geometric dilution method, which was introduced to me by pharmguy24 on AU. What's the geometric dilution method you ask? It's a method used when a small amount of powder needs to be *thoroughly* mixed with a larger amount of powder. To do this you mix all of the smaller powder with an approximately equal amount of the larger powder, this blend is called "mix 1". Then combine all of your "mix 1" with an equal amount of the larger powder and blend again, this is "mix 2". Keep going like this until all the powders have been mixed together."