Tapping threads straight can sometimes be a challenge. A lathe, drill press, or a lot of practice help the process — or you could use a tool like Big Gator Tools’ V-Tap Guide. A series of graduated holes in the V-Tap Guide keep the tap aligned perpendicular to the surface.
Big Gator Tools makes the V-Tap Guide from nickel alloyed steel, heat treats it, grinds the bottom flat, and chrome plates it. They also cut a V-groove into the bottom, which both gives the tool its name and allows you tap perpendicular holes on corners and round stock in addition to flat surfaces.
The Inch guide can handle most tap sizes from 0-80 to 5/8-18; the Metric version handles most taps sized 1.6 mm to 16 mm. Metric or Inch, either V-Tap Guide will run you about $20, or you can pick up both for $34.