From one of my previous posts:
For trim replacement it's easier with 2 people. I used electrician tape to mark on the opposite side of where each clip is. Open the boot and start at the bottom, loosely place the lower part of the trim over the boot/bumper catch and get into the boot. Have someone on the outside close the boot lid down till it's nearly shut and to keep the force gently applied as your about to push against it. From inside the boot go to the top of the trim, line it up and thump the electrician tape where the clips are. If your lucky they should connect and start to firm up the trim. Now work your way down hitting each clip in place until your at the bottom. Having someone hold the boot almost closed will allow you to force the trim on a lot easier than trying to hold the boot closed yourself