Hi, soundoholic,
I am sure my method is not precise. I of course have no direct knowledge of the Stax industrial process.
I aimed for right to left consistency and just stretched until no sags then tamped onto glue. I stretched the top edge of the mylar sheet with tape onto a glass refrigerator shelf, then laid down the bottom half of the metal bias/spacer, and pulled the mylar over it, when the tightness seemed right, let the edges hit the glue.
This is the ultimate DIY, just eyeing things as I went along, it is not a precise method. The 1.4 micron mylar is pretty fragile and tears easily, it will only stretch so much. So, just a combination of trial and error, instinct, and observation.
After they are glued, I spray them with anti static spray and let dry to as even a consistency as possible. I finished the drying process with a small hair drier. I then apply the top spacer, use a scalpel to trim, and put them back into the headphone. They sound similar right to left with a little tap to the membrane. As I say, I can only aim for right to left consistency, not industrial precision.
So far the ad hoc membranes are extremely high definition, much more so than the original 507, but also smooth and without any harshness.
However, I need to with hold any judgment until they have been in there a while without problems. If problems arise, I might try a thicker mylar.
It is an intricate process and I am sure that my method is quite flawed compared to the Stax process. However, I am surprised at the incredible detail of the new membranes, something I wasn't really expecting.