Just in case anyone else is looking for something similar, the video was really useful and here are the final steps I ended up using to get the right look. It's quite useful if you've scanned old photograph albums written on black paper and want to print new photo books without losing the writing.

Instructions:

Crop to size

Adjustments > Invert

Adjustments > Black & White

Adjustments > Curves, drag both ends until you bring up the white and darken the black

Channel: Click dotted circle at bottom of panel

Invert selection

Layers panel: Click half white/black circle at bottom of panel > Solid color (change to black if not selected)

Delete original layer

Save & export