The mixture should be light, seedling mixes are a good choice. My current favorite is a 50-50 mix of Just Right Xtra and Just Right Lite. Add enough water to make a doughlike consistancy, and let set for 1-2 hours. The mix should be able to be pressed into balls and somewhat hold their shape.
Allow the cubes to dry and they will firm up. However, they will always be a bit fragile, and are much easier to break than commercially made cubes.