This is what i usually print for the people i rent mine to. have a read
Go onto: goo.gl/xbeNg to see very useful Youtube video.
Firstly check the arch for rust, rusty arch will not roll very well at all, even slight corrosion will have an effect as it doesn’t bend quite the same as good steel would.
Next remove all screws/bolts holding the inner arch plastic if applicable. Then clean up the inside of the arch, behind the lip as it will probably be full of mud and road debris. A good stiff brush is usually enough. Once clean it’s a good idea to paint something like seam sealer, under seal grease or oil over this area as once the lip is folded over will be become a water trap without anything there.
Attach the arch roller to the hub with all bolts or studs just like you would with wheel and position the rolling wheel against the lip. You may need to jack up the axle so the roller will be in contact with the arch all the way across. It’s a good idea to use a hot air gun here to warm the paint up to try and prevent it splitting as it’s being rolled. Don’t warm it too much or the paint will go too soft.
Now add a little pressure by winding the arm towards the arch and gently move it back and forth. As mentioned before, the whole arch does not need to be rolled, just the area where the tyres might make contact – on the rears that’s just above the tyre but the fronts may need more for turning.
As the lip folds you will feel it becomes easier to move the arm, add more pressure and continue (and keep applying heat if you’re using it). Keep checking the angle of the rolling wheel and adjusting as necessary, it needs to be at an angle to the direction the lip will be folded.
When finished check the fitment of your wheels, if this has not created enough room then you need to move on to arch flaring, which is basically pushing the arch outwards with the roller. This requires a bit more effort and is not as easy to do a neat job, as you will be working with a double thickness (or more at the back) of steel where the lip is folded and having to stretch it.