well to drill and tap the hole you obviously need the right sized drill for the tap (smaller than the screw diamter), a tap and a tap wrench!
You also need some cutting lubricant and take your time.
You should easily find a guide for the correct size drill for your tap form where you get the tap from. I ogt a cheap tap and die set for £4 the other day from tool station as i needed to clean the thread in my brake hub before putting new disks on.
I have done a few threads in my time so here is what i do:
1. drill suitable hole using correct undersized drill - eg. for M3 you us a 2.5mm drill
if you need to do a large hole you need to build up with smaller pilots first - as our scotish friend suggested earlier
2. use the more tapered tap (point at the end) in a tap wrench with small bit of lube and place it int he hole and start to turn applying a fair amount of pressure. YOu have to be vary careful that you are going straight - so check the angle from all directions. Turn the tap a little bit at a time and check the angles. If it is out a bit then back it off and try again - pushing down a little bit harder to cut a new path.
3. When you have got the tread nicely started progress slowly, still applying downward pressure and every turn or so back the tap off a quarter or half turn to break off the swarf and kleen the threads
4. When you have reached the botton of a blind hole ot are in the nice full width part of the tap with in an open hole then care fully unscrew the tap and remove it - taps are VERY brittle as they are hardended to be harder than the metal you are cutting so be careful.
5. You normally get a blunt nosed tap in the pack, thsi is manily used for tapping to the botton of blind holes and you jsut simply use this next but be preapred to start cutting new thread when you reach the bottom where the other tap could not get too. Also be careful not to cut a new thread and cross thread the hole.
I normally put this second tap, when i have one, through open holes as well to ensure the thread is cut correctly and is not too tight.
If the body is too thin to tap correctly you might want to look for some expanding inserts you can just put in the hole from your side and screw into. You can bonf these in with araldide to make them secure.
Also don't forget to use the waxoil as mentioned and i am afraid you are gonna have to get underneath to was the exit of the holes as thss is where the rust will attack from.
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