There are a few things to keep in mind when purchasing your crabs. Be sure to look for healthy active crabs. They should be moving around and hide or run when you reach for them. If possible, observe them in the store and make sure they have been there for at least a week. This way, the weak and sick ones will have died off and you will be left with the stronger specimens. Remember to keep the male to female ratio at 1:2 or 1:3 to reduce aggression amongst males. Choose crabs that are similar in size, as large crabs may pick on small ones. Once you have selected your crabs, it is time to bring them home!
Have them packaged in some water, or they will dry out on the trip home. Try to make the fish store your last stop, so that they spend little time in transit. Keep your car at room temperature, not too hot or cold. When you are safely home, you can release them into your tank. Net them out and place them in your tank. Be careful, as they can run fast and might fall out of the net. Once they are in the tank, leave them in peace. They will take a while to get used to their surroundings, and your presence will cause them stress.
You can choose to acclimate your crabs before releasing them if you want. In my experience, they are quite hardy and used to salinity changes in the wild. However, if you wish, you can slowly add your tank water to the water they came in over a period of time (30min to 2hrs). You may also choose to quarantine them in a separate tank for a week or two to watch for disease, though personally, I think that is excessive.