Start with the pieces that will be touching the hoop itself (so the pieces furthest back in the layered design you've created).
Add a bead of hot glue to the flat part of the ring and then attach the dried pieces on top. Be very careful with your fingers, that hot glue can burn your fingers if not careful.
Once the bottom layer is attached to the wreath, continue glueing layers on top of that one until your design is completed.
Whenever possible, try to find areas to glue to that are the floral ring itself, which is the strongest spot to secure leaves and dried items. If that's not possible, look for flat areas in the layers underneath whenever possible.
This will help make sure everything stay secure.