What is the best way to get java moss to carpet the floor of your tank?

I really love the look of the entire aquarium being carpeted over with moss, I've researched and found a couple ways to make it grow onto a wall but does anyone know a good way to get it to carpet the floor of the tank? Does it need to be planted under gravel, or dirt, or can it just be on its own in plastic casing without any gravel or anything? I'm a beginner so I don't have too much knowledge on this subject just yet. I'd appreciate any tips or information you can give, thanks :)