Step 2: Add Cartesian coordinates

Draft a rectangle about the area you want to model, and choose two sides to become the +x and +y axes of a Cartesian system.
(These have no relationship to the parallels and meridians already on the map.)
The +y axis must be 90 degrees counterclockwise from the +x axis.
Distances along each axis must be real distances on the Earth (not distances on the paper map); for example, in this case the x axis is about 1.82E+06 m long, and the y axis is about 1.10E+06 m long.