how do you do it? You DRAW it!
It's hard to explain in words only, but
study any prismatic font, and you will see that establishing a center line is half the battle. Then at each termination of a line, the centerline splits to each corner. At each intersection or bend, centerlines run to the corners. On round sections, there is an "s-curve" of sorts that occurs on a common axis that defines where light will no longer reach a section.
I've seen articles in sign mags years ago... long before bevel effects were used in software, and I'm sure you can search out tutorials like that.
Years ago I hand drew the vector paths that create a prismatic look in Avant Garde, for my own logo. Once you understand the principal, it's not that hard.
You can use software tricks, but for cut vinyl, you should simply draw your own.