Flatbed is highly recommended.
Use best judgement where to hook up.
Don't pull from the tow hooks.
Use two pull hooks
Pull slowly.
Inflate the tires.
Double check car is not in gear
Unlock steering column with key
Be present when the car is picked up and dropped off, if you can
Secure doors, Hood and trunk before the truck leaves. -I forgot to, so I made the driver stop twice so I could secure the hood. Once on the freeway. He wasn't happy but I didn't care.
If you don't know how to tow, then just act really concerned and paranoid about having the car towed. The driver will usually be more carefully. A $10 too usually helps too.
I actually directed the tow driver what to do and even hooked up the car myself. He initially hooked on to the tow hook which would have ripped right off. The only place where I could hook up was on the rear axles, but he pulled slowly and all was well.
From experience, tow drivers often have a tight schedule and sometimes don't care if your car gets damaged.