Are these pants or jeans? When you say loose in the rear, how far down in the rear? Just up near the waist? Assuming pants and most of the looseness up at the top (which is most common), do this:

1. mark center back of waistband, up from the back seam
2. try on right side out, pinch back, mark at pinch on each side
3. pick out waistband stitches some inches past that mark on both sides (you need room to work) and also remove the two belt loops on each side of the center back - when you do this, note how the waistband was put on so you can replace it the same way. They vary, so if you have more questions when you get there, feel free to ask
4. cut piece out of waistband between marks and don't forget to leave seam allowances!
5. turn pants inside out, press to keep things aligned, then stitch from your mark and taper in to the existing seam further down, before the crotch
6. trim new seam, finish edges (not necessary, but very recommended), press so it looks nice outside
7. sew waistband back together, press nice and flat
8. put one of the loops on the center, throw the other one out, match center back seams of pants and waistband and reconnect, stitch top of loop down

It goes without saying that you can try them on at any point and see how it's going. If they're really loose, you may not be able to get it all. Take a good look at how they fit around that curve into the crotch and make sure not to sew yourself a wedgie. You might even want to mark where to end your taper if they fit better further down.

Good luck, I bet they'll come out better than you think.

Thank you very much. I'm going to give it a try and I'll let you know how it worked out!