1. If you pay for the shoot, then the organizer should arrange himself or someone to hold the reflector. The photographer just shoot.
2. If you are alone with the model, and you know how to do it, then use one hand to hold the reflector, and the other hand to hold the camera and press the shutter.
3. You can also bring along a reflector stand, and tilt it to the angle you want, but not very convenient.
4. If you only shoot model face, you can ask model to hold the reflector with her own hands.
Actually there are hundreds of ways to hold and use a reflector. You can also try using a while wall, sand and water, a white magazine etc.