A rosette is a strip of fabric or ribbon gathered or pleated into a round shape. It can be single or layered with other rosettes or trims, like beads, lace, silk flowers, jewels, etc. Rosettes were accessories that could be worn on almost any garment.
There are several kinds of rosettes, and a lot of them were used on 18th century clothes, hats and shoes. You can see one on a French Revolution hat here, I think it's ribbon, but it could be a metal medallion made to look like a rosette.
You can use ribbons, lace or fabric strips to make these. Ribbons work the best, and striped grosgrain ribbon works the best of all. In fact, any striped material will totally rock with this trim.
The first type is a round fabric rosette. Cut a length of fabric (the wider the strip, the longer you will need). I used about 10" of fabric to make a 3" rosette (1.5" strip). You need to have enough fabric to go around the outside of the circle without curling in, and enough to turn over at the ends. You will need more if you want a frillier rosette.
Finish the ends of your strip however you want, I rolled a small hem for this one.
Depending on the material, you can do one of 2 things.
1. Tie your thread at one end, make a running stitch along the "inside" edge, then pull the thread tight. The fabric or ribbon will make a nice neat circle and you can stitch the center closed from the bottom. Leave the ends of the thread if you are going to sew it onto a garment, cut them if you are attaching it to a pin or clip.
This works well for lightweight fabric or ribbon.
2. Anchor your thread at one end, make a very small pleat and overcast stitch it in place. Make the next pleat with a slight curve toward the center. Stitch in place again. Repeat. You may have to make several stitches to keep the pleats in place, but don't panic if they slip around a bit.
This works well for thicker fabrics or large rosettes. You will have to finish the inside with a button hole stitch, or put a smaller rosette (or bead) on top to cover the hole.
The next kind is a folded ribbon rosette, which I'm too tired to make tonight, but you can see a great video of it here. She's a bit sloppy, (because it's modern), but the first one she makes is one of the more commonly worn types of the 18th century. If your edges and folds are a bit neater, it's perfect. The cut bit she gets into is more along the lines of silk flowers rather than true rosettes, and is more 19th century than 18th.