Lay the ribbon out in a line and arrange flowers along it. Trim stems as needed and don't be afraid to deconstruct flowers. You can take out petals or turn one into two if you want.
Tie the tape around the back of the flowers inside the plastic backing or stem and tie firmly to ribbon. You can slide the flowers up and down the ribbon at this point to make them spaced out or crammed together.
Try on your crown before trimming the tape. Edit as needed and trim the tape. Take tons of pictures, post photos on instagram. But also, be brave and wear it in public!