I cut and rolled mine pretty much the same way as n_bannerman
I first taped em off in the radius I wanted, then cut with plasma (just try not to catch your fender on fire).
Once I did that I had a piece of masking tape on the lip to guide my bending, and I used a tool designed for bending sheet metal. When you bend the lip, try to only grip the next inch or so of the tab, so you don't end up with wrinkle marks between "grips", as well as bending only about 10* or so. continue this process over and over until the lip is bent as far around as you want.
This picture you can see the tool sitting on the leaf spring right next to the snips. the tab is only about half way through the bending
Here you can see the results of moving too quickly, and not taking time (wrinkles around the fender)
This fender I took a bit more time, and bent less at a time
and I was rewarded with less wrinkles in the fender.
As stated already by bannerman, rolling the fenders not only benefits you by not having a sharp edge to cut your finger and tire on, but also supports the fender so its not floppy