The master curve of an inverted image is (roughly) done with the following steps: Shadows, Highlights, Mid-tones, then final edit of Shadows.
1. Invert the image (Command or Control “I”)
2. Add a curve layer.
3. Pull the curve down in the shadows bearing in mind where the film base appears in the histogram. Don’t go too far here.
4. Pull the curve up in highlights to get a white(ish) point keeping in mind the highlight values in the histogram.
5. Pull your mid-tones down (usually) to bring general gamma from a linear 1.0 flat scan to something closer to normal.
6. Finally edit your shadow point as it has pivoted to the wrong place (often) when changing the gamma in the mid-tones.
You have created a master (or base) curve and are ready to do a tonal selection curve (next day’s email.)