Run a line of hot glue along the outer edge of the cardboard round.
Then glue down the garland.
Go round and round in an inward spiral. Mine took 4ish rounds.
Keep a'goin' until your mirror will fit in perfectly.
Then slap a glob of mirror adhesive in the middle to "glue" the mirror in.
Put something heavy on top of the mirror and let it dry for 24 hours (or whatever your glue says.)
I used a few inches of jute twine for hanging, just by gluing it onto the back.
Anyone who drools over Pottery Barn will love it - so it's a great gift idea!
All that style for less than $2? Ooooohhhh yeahhhhhh!