Inspired by a short instruction on the Mobileburn.com forum, I thought a more precise instruction would be useful.
Thanks to Moby at http://forums.mobileburn.com/showthread.php?t=12888
Step 1: Locating the clips holding the cover in place
Open the slider, place the phone so the rear side is facing up and note where the top clip and the upper side clips are located.
Step 2: Loosing the upper clips
Step 3: Loosing the lower side clips
Step 4: Removing the cover
Close the slide carefully, but stop if the slide is much harder to move. In that case the cover is loosening grip at the bottom clips and needs to be pulled back up for now.
When the slide is closed it can be removed entirely by sliding, not tilting, the cover furthermore downwards.