In a nutshell, the bit inside the tube is just jammed in, so yank it out, cut to size then jam the end bit in again.
I'll go over it in more detail in the following steps.
Step 1: Mark Out the Desired Length
Use a square to mark out the desired length across all of the tubes to be cut. The length I needed to cut was the width of my quick square so I just used it as a guide against the edge - not really the correct method of using that tool but it worked!
Step 2: Pop Out the Lugs
To get them out you could cut the tube and hammer them out from behind, but I prefer to attach the screw through a fixed point and yank. In this case it is fixed to a leftover shelf rail that I've clamped in a vice, but any bit of metal with a hole in it will suffice.
Once attached, gently pull and wiggle the tube until it comes free.
Step 3: Cut
Cut along your pre-marked lines. I've used my metal-cutting bandsaw, but a hacksaw with a bi-metal blade will do fine (it's just a lot slower, a lot harder and you need to be more careful about keeping it straight).
I then cleaned up the cuts on a bench sander, but some metal sanding paper on a block will work just fine if you are doing it by hand.
Step 4: Re-set the Lugs
I tend to go overboard, so at this point I turned it over, tapped the lug in slightly with a ball-peen hammer and then gently tapped the side of the tube end to rivet it in a bit better. This last bit is not really necessary and is likely to be the point where things get irreversibly screwed up, so it's up to you.
Step 5: Clean Up and Assembly
A wipe down with any old goo remover will get rid of any residue (I like the orange oil based stuff because it smells nice and I want to lick it afterwards - don't lick it).
Next, wrap some fine steel wool around the tube with one hand and spin the tube with the other hand to clean up any stubborn marks and small scratches.
Put it all together and screw it to the studs in the wall like it should be.
Optional step: putty over and paint the first lot of drill holes you put in the wrong spot. Idiot!