Ok lets get down to the down n' dirty tutorial. To make these bases you will need putty,a sculpting tool, hobby knife, sand paper, some water, 30mm (or 40mm) plastic bases and a mold. I've been using the afore mentioned mold for over a year now.
First press putty into the top of the base like so:
Next press the base putty side down first on to the mold. Press down hard, I use the palm of my hand and put a little weight on it.
Once the putty dries you can safely remove the bases from the mold. You'll end up with this:
I hope you liked this and found it helpful.