help with side marker install

An E36 has front corners that serve as turn signal and front side markers. Are you referring to turn signal repeaters mounted on the fenders behind the front wheels?

i am unfamiliar with the term "turn signal repeaters" but yeah the square lights that go on the fenders (i call them side markers) that seem to have come stock on some 3 series cars.

what I'm looking for is mainly a stencil or dimensions on the rectangle i need to cut out and measurements as to where i need to make said rectangle

Okay, those lights are not side markers which stay on low brightness with parking lights or city lights. They only flash full bright as repeat of turn signals to alert next lane drivers of lane changing on highways/motorways etc. This info is most useful in the wiring issues by the way.

Have you considered just buying the sidemarkers and then cutting the fenders at your discretion?

You're either gonna have to cut, or buy new fenders for a later car with them and then paint those ... either way, huge pain in the ass and not really worth it in my opinion.