Tapping threads into a hole? The tap inevitably wanders off on an angle. A good jig would be good but this one works ok and it's free.

Yes, you could just be careful and get similar results but being careful takes more time. Time is money. This way you can just jam the tap in there and be on your way.

Step 1: Materials

Thin steel slab ~1/8" is fine. Mine was a road sign from the scrap yard.
3/4" thin wood. Thicker would be better but I was worried about making it too thick.
Screws
Drill
Hacksaw or angle grinder etc.

Step 2: Screw Metal to Wood

The point here is to make a toughened guide for the tap. So we need the metal on the wood. The wood just serves as a soft spacer. The steel won't get eaten by the tap as quickly.

Blast some holes in the corners of the metal.

Line up at least one edge of the metal with the wood.

Twist down some wood screws through the corner holes.

Step 3: Drill Guide Hole

Now drill a hole matched to admit a bolt of whatever size you're working with.
Drill near the edge or corner so you can fit this jig in close to protrusions on your work piece.

The tap will slide in here.

Step 4: Using

I've found that a good way to align this thing is to slip in a tapered punch through the jig and the work piece's hole. Fiddle the punch so it's perpendicular to the work piece and clamp it in place.

Tap away.