Ive always wondered about those fender rollers and came close to buying one myself. I was always terrified id mess it up though
It's actually pretty hard to mess it up on our cars, especially on the back where the metal is really strong. If I had a newer car with paper thin sheetmetal, yeah, I'd be a lot more nervous.
The biggest thing when doing it is just to exercise patience. Don't try to force it or move the flange too much all at once. Keep working it while increasing the pressure as you go, and the flange will start to roll.
Once the flange has started to roll up, you have to start adjusting the roller head angle to continue bending it upwards as you go. It's mostly just about watching what the flange is doing, and making your adjustments as you go.
Nothing to it!