Step 1:
Use an old minox 36exp roll to measure length, if you want to get techy then get a board with indents that show you teh cutting range for different exposures or a guillotine type assembly as I've seen some minox users utilise.
Step 2:
Darkroom / cupboard etc
Wear cotton gloves, (can be gotten from chemist), Tightly spool the cut strip between thumb and forefinger of one hand and wrapping with the other, place this into the spool-less side of the snap lid cassette you have waiting threading the loose end 0.5inch-ish into the felt light trap so that it protrudes toward the other spool, snap the lid on, turn on the lights, stop sweating, have a cuppa etc, (it gets easier and less panic stricken each time)
Step 3:
In the light, unsnap the spool side, tape the loose end of the film to the spool with scotch tape, (I use this for all my bulk load, it sticks well and removes easily and cleanly), thread the film through the felt trap again and replace the snap on lid, done.
Load into the camera and enjoy.
n.b. if this is a new slitter i'd heartily recommend sacrificing a strip by dragging it through by hand after step 2, this ensures that it rolls through evenly and cleanly, my first home made slitter didn't and jammed on me halfway through a roll whilst out.