You just acclimate them, and put them on the substrate. Try not to expose them to the air much (so I've read). Make sure to move them if you treat the tank for anything that may harm inverts. I move them out for salt treatments too. Other than that, nothing. They feed from the water column. Take a good look at them on a regular basis to see if they're still alive. If they die and you don't notice, they'll foul your water.
Live ones feeding will have their shell cracked open slightly and you might be able to see their feeding tubes poking out a bit (if you have visible particles in the water you can even see the water flow)
If you suspect one of being dead fish it out and try to prise it open with your finger nails, a dead/very sick one will let you open the shell.
I'd make sure that they go into a tank that is well seasoned, if possible. Like kimmisic said, they are filter feeders.
If your substrate isn't that deep it'll be easy to keep tabs on it/them. I checked mine by using a turkey baster. I would squirt a jet of water at them and if they 'flinch' they're alive. I found this to be the least evasive way.