You basically look through their models and pick one. You email the owner to see if the model is available. When they confirm, they give you the address. You ship your items and they are supposed to return photos within a certain amount of time (each agency is different). You get free photos, they get free bows. They do allow you the option of asking for your items back, and you pay them in return.
OT, the problem with this is that everyone wants free bows, so suddenly every girl is a model and every mom is a "professional" photographer. It's really insane. Some of the pictures I've gotten back were seriously cell phone pictures. Be careful who you choose. Most agencies have a "modeled" album where you can see their work... I usually go by that, not the comp card