Welcome @roscka. You could use the same techniques that folks use for cutting paper, using an engrave to vaporize the material or just use the cut since you don’t need the surface to be pristine.
Yes, you would need some type of flat substrate or base to fix the mylar too. Folks often use rare earth magnets, but sometimes that interferes with the electronics in the head and can cause errors.
Folks also use a cutting mat, like a Seklema mat.
You could also use a piece of cardboard for a base and get some low tack spray adhesive. Spray it only on the cardboard and not on the mylar. It should release well after cutting. I use Loctite multi purpose spray adhesive and just to a light coat.
Glowforge settings are computing platform agnostic, so they are the same for whatever material. Here is one post about settings.
I understand that you are new and there is probably so much to comprehend. Keep searching and reading.