One would think that you need to figure out the flow rate of a stock can. Then corralate it the exhaust flow of your engine at set rpm points. Map all of this information. Next would be to design the new and improved version to meet the specifications that you have defined. Physical dimentions, Weight of the unit, Baffel design, etc.
Then you will need to build a prototype that you will then have to test and verify that it matches the flow rates that you mapped from the stock unit. Then you will have to test the unit in the real world. Does the new and improved design match the stock unit in performance, Sound Levels, durability, etc.
Then you learn from the mistakes, redesign and test again. This may take many steps just to match the performance levels of the stock unit.