Here is what I wrote as a PM to a fellow member a while back, maybe it helps.
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I frequently use some Rbk 9k shins I've sawn down, and didn't run into any issues. No special tools required, just stuff that should be around the house anyway. I'd recommend a small saw (e.g. from a leatherman) and a sewing awl / speedy stitcher (optional).
I'd do it again if I ever needed to. With Rbk/Jofa style pads you have a lengthy flat section at the end, very easy to cut down in that area. I personally didn't have to cut into the bulge/ridges that form above them, and I wear my shins over my skate tongues - if you remove too much flat section you might have troubles getting the shins to fit under the tongues.
The process was quite simple, I'd say do it as follows:
* Secure your stitches of the liner to the shell on the outside with superglue or similar, just apply a coat. Optionally you can cut the very last stitch, get the thread out of the way, and re-stitch once you're done.
* Note how the liner sits in relation to the shell and adjust length accordingly when cutting the shell.
* Don't just hack off X inch, but also compare the long shins with some shins that fit well length-wise and cut to whatever length is longer. You can always remove more material afterwards, but you can't add it back.
* Once you've done your first cut removing a bit of length, use some stick tape or similar to form a temporary skate guard. Keep shortening until you've reached your perfect length.
* Once you've reached your desired length, make sure to round off the edges well. You can also add a layer of fabric around the bottom of the liner (which you've also shortened), however I found this to be more for aesthetics than anything.
* Remove the original skate-guard / wrap at the bottom of the cut section of the shins and re-apply (no need to stitch, glue works fine) once you're satisfied with the length.