This is generally quite easy but it depends very much on which plants you're trying to propagate. Stems plants tend to be easy, just cut the top off and shove it in the gravel, it can be done with other plants just takes a bit more work, you could always phone greenline or tropica and ask them!
Well jimbo, cabomba is easy, but what about things like cardamine lyrata and alternanthera cardinalis . These two are both rooted plants yet dont seem to be growing new shoots. Is it a case of cutting half of the stem and replanting them?
yep, cut say 4 inches off the bottom and replant. even if they have no shoots they'll quickly regrow. if in doubt cut them and float for 24 hours, some roots will appear, then replant. (i just replant the stems and they have always rerooted)
for plain gravel substrates the root regrowth will be slower though and in that instance you may have problems. with laterite or simmillar it's fine.