Garter Belts & Elastic...

My method of attaching the red ribbon to elastic greatly reduces it's stretch-ability - but it's not a bad thing!

The elastic you find on most lingerie (like garter belt straps and bra straps) is typically a plush-back picot elastic. It is only intended to stretch about 40-50%. This is why most bra and garter straps also have an adjustable slide. If those straps were more stretchy then they could no longer provide the intended support of holding things up. You need just enough stretch to compensate for movement and comfort.

Braided elastic (middle of the pic) is typically what you'll find at the craft store and use in your costumes. It's far more stretchy; it's intended for waistbands and other such elasticized channels. It can stretch nearly twice it's length and is not suitable for bra or garter straps. 

However, attaching a ribbon (with a short, straight stitch) will greatly reduce the stretch of a braided elastic which, then, makes a suitable garter strap. The shorter the stitch, the more the stretch, but it will still be slightly less stretch than the lingerie elastic. Of course, the ribbon prevents attaching any sort of adjustable slider. 

But that's okay! Because the floorshow garter belts I sell are made from spandex. I cut them out so that they stretch more vertically than horizontally. This means that the garter belt itself is compensating for the rest of the stretch that the ribbon-stitched-on-elastic has lost. 

That all being said, there's no perfect strap length that will fit everyone. Some people prefer to wear their hose higher than others, different torso lengths, etc. Best I can do without fitting it on you to see where the garter belt will 'ride' is aim for an average length. The wearer can then adjust where the garter belt sits around the waist/high hips, and adjust the height of their stockings accordingly. My straps are shorter in front and longer in back (to accommodate the way the body moves). 

Floorshow Garter Belts

My best guess is the metallic red ribbon is attached with a zig-zag stitch to some elastic while it's being stretched.  This will make the entire garter strap curl up on itself, but you might be able to flatten it with an iron. Definitely test your ribbon/method with some scrap pieces first. 

Columbia's photo shows that the middle/side strap is attached on top of the garter belt (the other straps are likely sewn into the designated seam).

Rocky's garter belt gives some insight on the fabric - definitely stretchy (not a plain satin). 

Modifying Store-Bought Garter Belt

Modifying an off-the-rack garter belt is one of the easiest costume projects to tackle, so there's really no excuse (insert angry cast director or costume mistress rant).  Our cast's minimum standard only asked that floorshow corsets and garters be solid black, but it looks so much better when the performers go the extra mile with their garter straps and sequin gauntlets.  A spot-on kickline is killer.

The shape of the garter belt is a traditional 4-strap style with two side straps added.  And since all the straps are covered with red crinkly stuff it doesn't even matter if the added elastic matches.  You can order extra garter clips on eBay for cheap.  Make sure you secure your strap lengths before applying the red crinkly ribbon (wired Christmas ribbon works great and it's easy to find).

Floorshow Garter Belt

I used a heavy (bottom weight), 2-way stretch satin for this. There are two elastic straps in back - it just pulls on. The garter straps are 1/2" elastic with pleated, wire metallic red ribbon - and they still have a little stretch in them. I'll have a couple to sell at con.

Garter Belt (Research)

I suspect the garter belt is some sort of (semi?) spandex satin. It doesn't look as stretchy as swimwear, but it's not the same satin as the corsets. I've seen another photo of Columbia in floorshow where hers looks stretched, too.

As for the straps, my best guess is that the red is pleated before being sewn onto (what is presumably) elastic. At least that's the method I'm going to experiment with first. I have a few more screen caps from the pool scene, and those definitely look pleated.