I'll just repeat for emphasis -- don't try to fit shoulders to a relatively poorly planed (or chopped) mortised workpiece. Check it first. It has to be dead flat, to one whispy shaving hollow around the mortise. If you try to fit the shoulders to a bump (it doesn't have to be big at all) near the mortise where a shoulder will touch, it'll drive you nuts and take forever. You have to know what's out -- the shoulders or the mortised workpiece. The tool doesn't necessarily have a steep learning curve (though some in this thread do seem confused about how to use one), but fitting the joint at its final stages and understanding what to do and in what order most definitely does.
The mortise could also have been chopped at an angle creating a 'hard-side' and a profound shoulder gap that has nothing to do with the shoulders being off at all. They could be perfect and there'd still be a gap big enough to stick a No. 2 pencil lead in.
This video will help:
https://www.youtube.com/watch?v=XhtD14Qn5pM
Fixing the shoulders, in this instance, won't fix the bigger problem but just assures the unit will go up with twist but with closed, cosmetically perfect shoulders. That's a fail, in case you didn't know it.
This, of course, doesn't apply to those boffins who've never chopped a mortise at anything but 90* EXACTLY to the face. Listen closely to his acknowledgement near the end of the video that chopping them off-vertical does happen, and this is a more honest assessment of real workshop conditions as opposed to posturing on a forum about joints that ALWAYS fit straight from saw and chisel with no need for adjustment ("I haven't had to adjust a mortise and tenon joint since 1974" and all that garbage). When you read something like that, run, don't walk. It's bull$hit. You need a plan to fix the joint, rather than remake one or even two workpieces with an equally likely chance of having the same issues as the ones presently on the bench. Chippendale, Seddon, Sheraton, Gimson, et al. all fixed and adjusted little and not-so-little discrepancies along the way in everything they built. Nobody cuts every joint on every project perfectly.