Once you have your miter and bevel angles determined, it's time to set your saw and molding up for a proper cut!
The image above shows the setup for a left inside corner cut. Your miter angle will be to the right and your bevel to the left with the top of your molding against your saw fence and your molding laying flat. You will be keeping the material to the left of your cut.
For a right inside corner cut, the image above shows your proper setup. You will set your miter angle to the left this time and you will flip your molding so the bottom edge rests against the saw fence. Your bevel will sit to the left and you will be keeping the material to the left of your cut, just as with the left inside corner cut previously.
For my process, regardless of whether I am only making one fancy edge or two, I prefer to miter and bevel both edges or miter and bevel one edge and square the other, then check for proper fit before I begin the coping. Coping is a tad exhausting and so I would rather adjust for length and fit before I take the plunge and then have to redo it if the fit is off.
When you are measuring for length on pieces that will be for the other walls aside from the first wall, I find it helpful to measure both the short and long length, just to give me a guide. The dimension you will definitely need is from the bottom of the molding you have already installed to the opposite edge of the wall or to the bottom edge of the molding on the opposite wall (depending on your installation option). Basically bottom edge to bottom edge since that will be your long length, so to speak (to get a visual of this, refer to the step above with the direction of the cuts diagram).