Spray roses are popular for cut flower designs because of the smaller heads. They are sold ten to a bunch versus the 25 to a bunch that standard (regular sized heads) are sold in.
Spray roses tend to open wide quickly. Don't be surprised on a little browning on the outer petals. Like standards, these are the guard petals that need to be removed before creating your corsage.
Wash your hands and spray with a sealant like Finishing Touch. This helps prevent the brown spots that appear on roses when oil is transferred from your fingers to the flower head.
Gently peal off and remove the outer petals that aren't perfect.
You'll want to take advantage of all sizes of your spray roses, with using the full blown ones in the center and the smaller heads and buds on the outer edges of your corsage.
Fully opened spray roses can hold up longer than you think. Just be sure the flower feels firm to the touch.
Petal transparency and petals dropping are signs that the rose is aged and may not hold up in a corsage (especially since there is no water source).
For the center of the corsage, I tend to cut the stems very short.
I apply Oasis floral adhesive generously so the rose will adhere firmly to the silicon pad when dried.