Most artificial flowers come in bunches or bouquets. Begin by disassembling the bouquets. Be certain to keep the leaves as well - they make beautiful accessories. Remove the flowers from their stems. Finally, each flower will have a small "nub" where it was attached to the stem. Take your scissors and carefully remove that nub. This allows the flower to lay flat on the headband.
If you have a wig head you can use that to stretch your headband. If not you can use a stiff hat or something round. With the headband stretched begin glueing flowers, as you like, to the headband.
You can layer smaller flowers on top of larger ones or spread the flowers along the headband.
You may want to add some ribbon. If so, simply fold your piece of ribbon in half and slide it under the headband.
Bring the two ends of the ribbon through the loop, tieing it to the headband.
Pull to tighten and secure. Now the ribbon will hang from your headband.
All done - add as many flowers as you like to create your own piece of wearable art.