Placement of clam: Place the clam with the mantle up towards the light. Alot of times you will get your clam already attached to rock or a clamshell, or something else. This may or may not make your life easier depending on your tank. You can either place the clam somewhere and observe it for signs of good health (mantle expansion, not trying to move), or you can cut the byssal threads.
This is done by poking, prodding, tapping the soft tissue surrounding the byssal threads. If all goes well it will retract and reveal the byssal threads. Take a sharp instrument, and cut the threads. You must be careful not to pull the clam from its attachment, or irreparable damage may be done to the byssal organ, in which case your clam may be a world of trouble. I have heard of some croceas and maximas living after losing their byssal gland, but the chances of survival are slim to none IMO. After the threads are cut, place the clam where you think you want it to be. Don't wedge it into a rock, or it will not be able to open fully.