I think it's really up to you, and what suits the image and style of card you are aiming for. I don't know that I'd necessarily bother with a frame at all - in this card I just matted it as you would any other stamped image:
WSC39, Seaside thoughts by Cook22 - Cards and Paper Crafts at Splitcoaststampers. As you can see in the write-up I just used spray adhesive to fix it to the backing paper.
You could consider using photo corners -
SC253 Acetate Bulrushes by Cook22 - Cards and Paper Crafts at Splitcoaststampers, or mounting it behind a frame cut with Nestabilties..
On this card
WT291 Simple Floral by Cook22 - Cards and Paper Crafts at Splitcoaststampers I used a leafing pen to edge the acetate with silver.
You can also incorporate brads or eyelets into the design to attach it to the base, as with vellum.
Really there are no rules, play around, have fun and let us know how it turns out
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