You'll want to wait until your flowers *just* start to die. Enjoy them while they're alive until they start looking a little sad.
Step 1: Supplies
Step 2: Get rid of the ugly parts
You'll notice that at this point the rose is basically at a 90 degree angle to the stem. Hanging it upside down will fix that right up.
Step 3: Cut your string
Step 4: Tie string to flowers
Step 5: Hang
It's very important that wherever you're hanging them is relatively dry. Don't hang these in your bathroom to dry out. Once they are dry, they will still need to stay somewhere kind of dry, so nowhere too humid.