Also, when laying out your holes make sure you get them centerlined to the receiver so that the scope mount is as close to your zero point as possible.
Tapping for a side mount on an SKS, you're not really going to have a problem with the windage, but yif the holes get off track your elevation may be too far off to dial in your zero with the scope's adjustment.
To get holes tapped straight, most of the time I use the drill press.
I chuck the tap into the drill and turn the chuck by hand (do NOT use the motor!
Usually, I will tap each hole right after I drill it and before I move onto the next hole. That way you don't have to re-center the work under the tap...small taps don't take very much side-stress before they snap off.
Break your chips frequently (Do a tiny turn, then back off. Another tiny turn, then back off, repeat as needed.) If you cut too big of a chip, it'll curl around inside the flutes of the tap and jam it up...and snap off.
Use the right size drill bit for the screw/thread size you're cutting.
Google search for "tap/drill size chart".
There is a very complete one on Wikipedia, but it's not a handy one to download or print out. http://en.wikipedia.org/wiki/Drill_and_tap_size_chart
Lots of them out there that will cover any size hole that you'll be tapping.