If you're building your own exhaust, you either need a tubing bender, or great welding skills with a quality bandsaw. And LOTS of time!
Pie cutting your tubes to make the sweeps and bends and get equal length pipes, merge collectors, etc is an enormous undertaking. Then, welding it all up without tremendous distortion to maintain fitment. Sheesh! Most folks who make systems have jigs to hold pieces in alignment while welding (for good reason).
Or, having decent software to calculate your bends, a tube bender and mandrel with some oomph, and full scale print out to lay your pipes against for checking, still need to fab and weld the collectors and flanges on. Sure you don't want to buy one instead?