I can tell you how I do it. A lot depends on what resources are available. If there is a schematic available, I use that as a guide and get the general setup done.
Then I'll generate all sorts of test tones to see how it reacts to the test tones, and tweak the algorithm so that the plugin matches hardware on the test tones.
Then I'll take actual music and make sure it sounds the same, and tweak it until it does. I tweak until I can't pick out the hardware from the plugin in a blind test, then pass it on to others to make sure they aren't hearing a difference either.
I bounce back and forth between those 3 steps until you end up with a plugin that sounds and measures the way the hardware does.