Decide how big of a sheet you’d like. A little goes a long way, and it’s easy enough to make more later, if you underestimate it!
Cut a piece that size out of both the metallic and the glitter Heat Transfer Vinyl.
Position the pieces of heat transfer vinyl together, shiny / glitter sides facing out.
Following the directions on your HTV, iron the pieces together.
For this brand, that meant having the iron on the “Cotton” setting - no steam - and pressing down for 10 seconds, but the brand you use may vary from this.
Allow your sheet to cool fully, on a flat surface.
If there’s a curl to it at all, set something flat and heavy on top of it, while it’s still hot.
Peel the plastic sheet off the glitter side, if you’d like.
Leaving it on gives more stability, and can protect the underlying garment from snagging on the glitter (if it’s something fragile like chiffon).
Peeling it off gives it a more flexible structure, and more sparkle.