The downside to casting bases is the upfront cost. Without coupons it runs about 60 bucks, but if you want to split the cost among your gaming group, it isn't so bad. I am using all alumalite products.
Step 1: Cut a circle out of plasticard that fits into a base.
Step 2: Sculpt the base using your material of choice. I am using yellow-grey Milliput here because it is much cheaper than green stuff. I have been trying out some polymer clay as well, but haven't made any molds with them.
Step 3: After you have sculpted your positives, glue them down to a piece of plastic/non porous surface and mix up the silicon mold material. Follow the directions on the box. I used the top of a McDonalds cup to form the rim of the mold. I fit 4 medium bases into it.
Step 4: Cast the bases and paint them up!