Tapping threads into braze-ons
It shouldn't be too different to tapping out anything else, the added difficulty being that you would normally clamp the workpiece in a vice and you don't have that option.  I'd take the wheel off and lie the bike down on a bench with scrapwood in place of the axle (try to clamp/screw it in place) to give you something to push against other than the frame.

Is the frame alloy or steel?
Especially if it's steel you really need to make sure the pilot hole is exactly the right size first - which probably means buying a drill bit specially. 
Getting a tap started in steel isn't all that easy especially as you can't get the frame in a vice, in alloy it's not too bad - so a brand new decent quality tap is a good idea for steel,as is just smoothing the edge of the pilot hole with a bigger drill or countersink tool before you start. If you're buying the tap, get a taper type - they start much easier than a second, and you've got plenty of room to come out the other side with the wheel off.  A decent tap wrench is worth the extra money over the junk that sometimes comes in cheap tap/die sets: you want the tap to be held securely.

Even if the pilot hole is already the right size for your tap, running a drill down it to take the paint off won't hurt.

Chain oil should do as a cutting fluid if you don't have anything better, then once you've got a thread started remember to back the tap off every now and then (e.g. forwards half to one turn, back a quarter).

Make sure to clean up all the swarf afterwards so it doesn't get anywhere important.

I assume the chain is catching on the nut rather than the bolt itself - you can buy half nuts / lock nuts which are thinner.  You'd need to threadlock it on.