The first thing I do when I make a mini is make a base. The typical D&D mini has a base 1inch in diameter. Larger monsters have a base that is a multiple of that such that it fits on a grid with 1x1inch squares.
1. Pick up a piece of sculpey: The firm variety is the best for bases because it's easy to smoothe out after you've scratched it during the sculpting phases.
2. Work it until it's softer and then roll it out to a couple of milimeters in thickness,
3. Use something that's 1 inch in diameter as a guide for the size. I used to use a Canadian 25 cent coin, but now I use pill box with a hole in the base.
4. Cut off the excess clay and smoothe out the edges: Flipping it over before smoothing is often helpful.