Having done this a few times now, and forgotten exactly how, I thought I'd document a few useful tips.
I somehow regularly manage to catch the rear lower edge of the drivers door and pop the end of the black bottom trim off its fixings. Just did it again recently and fixed it today, so here goes:
1. There's a thin trim strip on the inside of the door that covers the securing screws, clipped over some metal mushroom heads. In theory it slides off backwards with the door open, in practice it CAN be levered of gently with a trim tool if it really won't move. When refitting it should slide back on if cleaned and pushed carefully.
2. Removing that strip will reveal two screws (one each end) securing the outside strip. Remove these, they fix into the end fixing clips of a row of several.
3. The strip can then be slid backwards a few mm (may need a tap) to release the row of fixing clips and should come free. The clips have keyholes that slip over lugs on the door. Failed fixing clips may be detached from the strip and be left on the door. If slid off the door they can usually be reused unless badly damaged.
4. The detached fixing clips should be refitted onto the strip, ideally not at either end. They have two small lugs on one edge and a large "leg" on the opposite side. Fit the "leg" in the small slot first (you may need to reshape the slot) then push down to engage the lugs on the opposite side.
5. Ensure the fixing clips at each end are not broken, the screws go into them. Swap if necessary. To remove a fixing clip ensure it is warm and soft (indoors or use a hair dryer if cold) and lever gently on the two small lugs - you may need extra hands and two small flat-blade screwdrivers. With that edge released the clip swivels up and the larger "leg" should come free.
6. I've found that clips tend to have a break in the "leg" but those clips can usually be reused as long as they are not at the end and have complete clips either side of them. As long as they don't just fall off they should give enough support.
7. Refitting is as ever the reverse of removal and usually quick and easy. Just make sure every fixing clip is engaged on the door lugs before pushing the strip forwards and do that in stages to ensure the screw holes line up - you can easily push too far. After fitting the screws a tiny dab of wax on the inner trim strip lugs will make that easier to slide back on.
I've never had to replace a rear door strip but expect it's similar.
