Usually, you can buy moss spores (hopefully the kind that do not need a dormat period)and sprinkle them over some starter soil. Then cover it with plastic and keep it in a area thatgets morning sun and afternoon shade. If you want it to grow on a branch, try to soak it first and then put it in the same set up. This is how many Bonsai enthusiasts start the kiyoto moss that is frequently used in bonsai pots. One source is Jsua roth, I buy from them. There is something called java moss that i see used often in vivs but I have no experience with that.
Hopefully someone has more experience than I do and will help you out more.
Nick