Flat field creation is a simple procedure. You should already have loaded the relevant file names for red and blue flats into the proper fields, and set the red/blue transition row in the Preferences pane. If you're not sure what this last sentence means, refer back to the "setups" section of this wiki for a refresher.
Then, click "Make Flat". The code will first combine the red and blue flat fields in 2D space, using a smooth weighting function to spread the transition over a span of ~150 pixels. After masking out pixels not in the slit, a bspline is used to fit out the wavelength dependence of the flat field, taking the slit curvature into account (this is why arc solutions must be generated before flat fields). The map model is then divided out in 2D, leaving a ration image centered about 1.00 with departures indicating either bad pixels or gain variations. The final flat is presented for viewing if requested in the Preferences pane: