Kenny, just to be clear, the size you tap to has to match the hole drilled (and as Chris says, the drill size will be stamped on the side of the tap). If the 12mm tap doesn't call for a 27/64 hole, you need to either
1. get the tap for a 27/64 hole
2. get a tap and drill bit for the next size bigger than 27/64 and drill them out some more.
You choice of tap should also be driven by the size and thread bolts you can buy.
Then to actually tap out the holes, you need to line up the tap as straight as you can and slowly turn it into the hole. As you make progress cutting the thread you need to back the tap out (turn anti-clockwise) and then turn clockwise again repeatedly. This cleans out the thread you just cut and makes it easier to cut more. Use oil to make the process go easier.
Oh, and be sure to wear eye protection since you'll be spending more time on you back directly under the holes with crap falling on you. Music helps. Children "helping" usually don't as you'll have a short fuse at the time.