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.
I’ve had good luck with Rapid Rooters from General Hydroponics. They are used to start seeds or cuttings. If they dry out before use, you can re-wet them.
They work well as long as they are kept moist, but not soggy.