With most moss, what my old LFS used to do was a painstakingly tedious task, but results always looked fantastic...
1. He would first separate every strand from a large clump of moss and lay it out on some soaking wet newspaper in a shallow dish with just a thin layer of water to keep anything from drying out
2. He would then find either a piece of driftwood or slate he liked and then start laying down individual strands (all going one direction)
3. Once he got the first layer complete covering the area he wanted (and i mean completely so you dont see anything but green), he would tie some fishing line around the the wood or slate making sure all the moss was held down firmly...
4. After the first layer, he would sometimes do a 2nd thing layer if he felt the need to, otherwise, he would then place it in the tank and let it grow...
If done correctly, it will grow and eventually start to spread... i think normally the first month or two will be slow, but once it settles and gets going it can turn into a huge clump of moss in a matter of 3-4 months...