Please try the following steps.
Check if you have all the pieces needed for installing the clip at the back of the cover ready: a silver round-shaped piece, a screw, a sticker and a star-shaped clip.
Place the round silver piece on the inner part of the back cover and the star-shaped clip on the outer part. Use a Philips head screwdriver to fix the 2 pieces, so as to attach the clip to the back cover.