There are a few ways of doing this, ranging from "just drop them in saltwater" (not recommended) to drip-acclimating for a few hours to a couple weeks. I've had good success (once) acclimating 3 mollies overnight. Took me about 14 hours. I just made sure the drip was slow, but not too slow.
They're doing fine and bred soon after being placed in the tank. Babies are about 3/4" now (all white) and have lived their whole lives in full-strength SW (1.025).
(Edit: Acclimating for a couple weeks was done in stages, not a continuous drip

Additionally, i've heard of guppies being acclimated to full-strength SW too. They're reportedly more delicate than Mollies, but can take it. No personal experience on that one.)