I’m trying to create a custom CRS in QGis that basically is a regular Cartesian coordinate system.

I have a scanned map ( raster ) with 3 known points on it ( vector ). The coordinates of these points are known.

I’m trying to find the origin point of the Cartesian coordinate system and convert it to a custom CRS , QGis can understand, georeference the image to that custom CRS and add more points to it to find their coordinates instead of pulling out the ruler in real life.

I don’t know where to look at, where to start or what the prj code line means in QGis when you’re defining the custom CRS. Any help is appreciated.