I have mounted a lot of 35mm slides manually as this was a service in a lab where I worked - fortunately most rolls were unmounted. Anyway, we would cut them with (sharp) scissors over a light-box, making say a dozen individual transparencies on the box, then give the glass mounts a sharp tap on the workbench to dislodge any fluff before sliding in the edge of the film and closing the slide. Note that each frame has eight perforations and the end of the frame should be between the perforations - if the camera is mis-adjusted then you will see it somewhere along the roll and so can still decide where to cut on any very dark frames. We always used GePe mounts as, at the time, they were the best available, for ease of mounting and reliable projection.

It is a good idea to do everything you can to avoid creating static-electricity around the area of work, and you might also want to use one nitrile glove if you have sticky fingers. The whole process is quick and straightforward if you are organised.