I'm sure others do it differently, but here's what I do...
First thing, I whip out my refractometer and see just how far off the salinity of the water the critters are in is from the water in my tank... I adjust the length of time for acclimation accordingly up or down...
I use a plastic pitcher, rubbermaid container, non-metallic caserole dish (My wife just LOVES it when I use one of those for acclimation!) and pour the critters into it with their water...
From there, I take 1 turkey baster of container water out and discard it, then add 2 turkey basters of tank water to the container... Repeat this procedure every 15 minutes or so until the water in the container matches the salinity of the water in my tank...
Oh, and if the container starts to get too full I just take 2 turkey basters of water out of it...
Snails are very suspectible to ammonia, but they're also very suspectile to osmotic shock from rapid salinity changes, so you have to balance both needs as best as possible...
I'm sure someone else will pipe up with a far better method, but that's how I handle it...