I recently needed to convert my (straight) mic stand into a boom stand. I did it using some PVC parts. The total cost was about $7, but the knobs and PVC cement accounted for about $5 of that. It took me about 30 minutes to assemble.

Note that you can get an actual boom arm from musiciansfriend or the guitar center website for about $10, but I didn't want to wait a week for shipping, and this was cheaper anyway.

Step 1: The Original Stand

This is the original mic stand. The upper arm telescopes in and out of the lower arm and is tightened with a twistable clamp.

In the picture, the upper arm is retracted all the way into the lower arm.

Step 2: Parts

Here are the parts I used (I brought my stand to the hardware store to make sure everything would fit):

1x 3/4" PVC T fitting
1x 3/4" PVC elbow fitting
1x 1/2" PVC connector
2x #12 O-ring
2x Thumb screw

Not pictured:
Drill (with appropriate bit for thumb screw)
A wrench (to get leverage on the thumb screw when tapping the threads into the PVC - possibly necessary)
PVC cement (probably necessary)

A note on the O-rings - I used them as a buffer between the T fitting and the boom arm, since my boom arm was jiggling around in the PVC fitting.

Step 3: Bracket Assembly Part 1

The pieces will basically fit together as shown in the first image. The bracket will pivot between the 1/2" connector and the elbow fitting.

You will probably need to cement the 1/2" connector into the T fitting, because when the bracket is assembled, the boom arm will put a lot of torque on this joint, and even jamming the 1/2" connector into the T fitting will not be secure enough to prevent the joint from coming loose (this was my experience).

Step 4: Bracket Assembly Part 2

Drill a hole into the T-fitting. Use a bit small enough that you can tap the hole with the thumb screw. Place the hole on the "long" part of the T so that the thumb screw will tighten into the boom arm when the bracket is assembled. Screw the thumbscrew into the hole. I had to use a wrench on the thumbscrew to get enough leverage to tap the hole initially.

Step 5: Bracket Assembly Part 3

Drill a hole into the elbow fitting as in the previous step. Place the hole close to the end of the fitting so that the thumb screw will tighten into the 1/2" connector when the bracket is assembled. Screw the thumbscrew into the hole as in the previous step.

Step 6: Final Assembly Part 1

Put the O-rings on the boom arm (the "movable" upper part of the original stand). Put them close together so the T fitting will cover them when it's in place. Put them far enough apart so the thumb screw will tighten into the boom arm between the rings (i.e. you don't want an O-ring to be smashed between the boom arm and the thumbscrew).

Step 7: Final Assembly Part 2

Put the boom arm through the T fitting. Position it over the O-rings as described above. Tighten the thumb screw to secure the boom arm in place in the T fitting.

Step 8: Final Assembly Part 3

Put elbow on the lower mic stand (the part that doesn't move). Push the T-fitting-1/2"-connector assembly into the elbow. Rotate it to the height you want. Tighten the elbow knob to secure the bracket.

That's it!