why would you want to is an excellent question. i know how to trap a spirit in the grave, but not a crystal. it would be neat to add one to my BoS but i don't know why anyone would want to do that.
perhaps you could trap a very weak spirit, but i wouldn't exactly know how. if there's a way to get it to go inside then quickly do a binding spell on the crystal, but i'm not sure.
i'm still a little stunned someone would want to trap a spirit. if you want it to help you, you can just ask. spirits are all around us, they aren't exclusive to old weather torn buildings. they wander the earth and when they find a place or person they like, they'll stay. if you come across a spirit attached to an object, it's bound to the object, so you could use that, but i DO NOT suggest doing that. on one hand it's horrible, because you're blackmailing the spirit, second, those spirits tend to be strong. if you take their ring for example, they'll get it back by going through you.