yeah, I don't know how other people do it, but how we usually do it is literally squeeze the fender together(front to back) which pushes the middle out.
It was done to this car when my friend Eddie(drift kid) picked it up, because it had a huge set of centerlines originally on it.
specs: 17 x10 +20 front with 25mm spacer 225/45/27
17x10 +20 rear w/ 50mm spacer 225/45/17
And so I don't break the rules, my car:
front: 17x9 +12 215/40/17
Rear: 17x9.5 +12 235/40/17