A newer version of this article, with full photographs of a bag under construction, is here.e
People seem to be asking, so I'll share. Unfortunately I have no pictures for this, not at the moment.
I've seen a number of variations, and I've seen a number of failures. The following combines the best aspects of several designs I have used.
Tools; sewing machine, 1 1/2" elastic, 1 1/2" and 1/2" velcro, about 6" of water-repellent fabric per two packs. White is the standard (it seems to hide best under costumes).
Cut a simple pouch; double the fabric over, lay one of the larger transmitters you have (like a Shure SLX) on it and trace. Allow 1/2" for simple seams, and build it 3/4" tall. Allow a 1/4" margin on both sides if using non-stretch material. Stretchy material does not need to be cut on the bias. Stitch along the sides, zig-zag the edges if you really want to, turn inside-out.
Remember, you are putting a condom-wrapped transmitter in here, and you may tuck some of the extra microphone wire in here as well. So snug is good but tight is not.
Attach male and female velcro to close the mouth; one continuous 1/2" strip (you can leave a small gap at the sides.
Now cut the 1 1/12" elastic to length. If you have the waist measurements of cast, cut to that, otherwise make a selection in several lengths. They can always be pinned up in use. Mark them with a Sharpie with their lengths. Put a good 2-3" of velcro on either end but don't compensate for the overlap; the overlap of the velcro is what takes the slack out of the elastic when worn, and the length of the velcro patch gives you adjustment in how tight it is.
Carefully working through the mouth of the bag, stitch it in the top and bottom corners to the elastic. Where on the elastic doesn't matter; anywhere between centered to 1/3 mark.
Top and down, however, it is best if the mouth of the bag is slightly "above" the elastic, but most of the bag hangs "below" the elastic (as worn.) If you are good with the machine, or have patience for hand-stitching, stitch the ENTIRE width of the elastic. The second worst mistake people make in these belts is getting lazy and stitching only the top. In use, then, they flop around and can even flip over.
In use, the pack slides in the pouch with microphone wire (and antenna if any) on top. (Some older packs, like the Shure LX, have a bottom-issuing antennae. You need to build the pouches for this with a stitched button-hole at the bottom of the pouch.) The velcro just closes over everything. Then the elastic goes around the actor's waist, usually at gut height or higher (right below the lower margin of the ribs is comfortable for many).
Often, centering the pack in the middle of the actor's back works best. Some actors with more physical roles will chose to wear the mic belt inside out, with the elastic holding the transmitter firmly against their body. The belt should also be worn OVER the most UNDERNEATH layer of clothing; preferably a layer the actor will never take off during the performance. The microphone cable travels along this layer, exiting at the shirt collar.
For less demanding physical roles, an alternate pack design uses a stitched fabric tube for a belt instead of a single length of elastic. There is no elastic in this design; just the velcro close. A fabric with a slight give to it is preferred! Cheaper 1/2" elastic can also substitute. These make up the sleazier mic belts you will find in stock at many places. They are usually stitched so the entire bag hangs down from the belt. In an active role it will flop around and can even spill the transmitter out entirely, but for standing and singing it is fine.
And there are commercial packs. These tend to be extremely robust designs in thick rubberized fabric. If you can't find any at a local theatrical supply, look for aerobics instructor supplies.