I use lenses modified this way on a Canon EOS 5 film SLR, attached in this manner they're fully manual for both aperture and focus as the camera can't talk to them, and it's your own eyes that will be doing the focus confirmation - just as it always used to be.
My camera is perfectly happy with this and will figure out it's own shutter-speed to suit whatever aperture you manually set, or alternatively you can switch to full manual and figure the shutter-speed out for yourself for more control.
The same basic principles will apply to any other type of lens you can get a suitable adapter for, but couldn't normally use on a full-frame camera because of stuff sticking out of the back.
Please be gentle with me - this is my first Indestructable. :-)
Step 1: What You'll Need:
A K-mount lens
A tiny cross-head screwdriver
A lens-cap or similar container for the tiny screws
A mouse-mat or similar to stop things sliding around
Some cutters that can cut both metal and hard plastic - I used a pair of pincers
Step 2: Dismantling the Lens
Place the lens on the mousemat with the end that normally goes to the camera facing upwards
You'll see a few tiny crosshead screws, remove those and keep them safe - try not to drop them on the floor or you're going to curse trying to find them. Don't ask how I discovered this...
Once the screws are all removed, the plate with the bayonet fitting should be able to lift clear - be careful as some lenses may have spring-loaded parts hiding to poke holes in the eyes of the unwary... mine didn't so all was ok.
Step 3: Cutting the Lever
Cutting the lever
I cut the lever by nicking the sides as low down as I could reach, then simply twisting it off. It's butchery that's taking place, but as it isn't going to be visible the finish doesn't matter. Be careful that you only twist rather than pulling otherwise you risk dismantling a bit more of the mechanism than you intended!
Step 4: Cutting the Guard
Make sure you get rid of any loose chips from the filing, you don't want then ending up inside the lens when you reassemble it
Step 5: Reassembling the Lens
On my lens, the hole-spacings for the tiny screws weren't all the same so the plate would only fit in one position.
This is good, it means I'm sure the markings will be at the top where I can see them!
Step 6: The Finished Result
Enjoy bokeh heaven with an easy supply of fast prime lenses at prices that are a fraction of modern auto-focus lenses, and often perform substantially better than a zoom-lens.