Consistent with the manual, entirely remove the slide stop from the frame after you have aligned the witness marks. Then let slide move forward a bit under recoil pressure, and do the following: pull trigger and hold it back and simultaneously tilt the muzzle down at a 45 degree angle- the slide comes off on its own. It's similar to the XD in that sense. 'Catch' it as it slides off the frame.
I've found it easier than reassembling slide to the frame.