The official line is to quarantine them in fresh, dechlorinated water for 30 days to be really sure they are free of pests/bacteria. Some people also clean them with certain chemicals to eradicate any unwanted snails etc., but I won't go into that because I'm not sure what they use and what is safe
Some people rinse the plants in fresh water and check over for snails etc. and then soak them for 24 hours or so.
Others who are much more impatient and lazy (me) just check over the plants and chuck them in with the axies! (no problems so far!)
It all depends how careful you want to be and if you are confident that the place you got them from is trustworthy.
I got one lot of Java fern and moss from an aquatic supplies store where the plants are kept seperate from any fish (important so that you don't transfer any diseases), the other lot of Java fern and moss my brother gave me from his tropical fish tank (I trust him).
