Here is how I created the bases for my warband.
I used 2 part Alumilite Silicone RTV to make my mould. It comes in 1lb packages, 14.4 oz of silicone and 1.6 oz of catalyst. Its a 10:1, which are what the measuring cups are for. 2oz of silicone to one *portion* of catalyst, which is what the little cup and scoop are for.
Here is my positive, of which I will make a negative - my mould.
I used a small amount of superglue to fix the bases to the tin, then I did a displacement test to see how much silicone I would need to fill the mould. The bigger the mould the more important this becomes, but it comes down to volume, which is l*(h*w). This is easy when your mould doesn't have round corners. In the same token doing a displacement test on a larger project becomes more work than worth since you have to have a completely dry positive. I used my silicone cup and filled it to the 1oz mark, then poured it into the positive, repeating it until its full. I need 3oz of silicone RTV for this mould. The tin is deep enough so that the highest point of the piece is covered by .25" of silicone.
I mixed the silicone and poured it, and then left it over night. Here is the finished product.
Once you have a mould you need something to put in it; in this case two part epoxy resin. Following the instructions on the kit, I microwaved the mould until it was 140ºF, then mixed equal amounts of the A and B halves of the resin for 25 seconds. Then I poured into the mould. The mould has to be warm to offset the heat generated by the resin curing. In these small square bases, I don't think it would matter too much, but in large more complex pieces, or thinner pieces the casting heat can cause warpage in the pieces. After three minutes, the new bases were finished and I popped them out of the mould.