Terrapins are easy to handle, since they tend to pull into their shells. Simply grasp them by the sides. Catching turtles isn't hard at all. The lazy man's way is to use jug lines. A large fish hook is snapped onto a wire leader, and the leader is attached to a plastic jug by a length of line.
Chicken livers make a good bait.
These are then dropped into the pond or lake of your choice. They can be left overnight, to be retrieved in the morning, or they can be tended from a boat during the night. If you decide to leave them overnight, make sure that you have some way of retrieving them. Another way to obtain your turtle is with a trap. There are two basic traps.