1. Look for a triangluar shape that is slightly darker and redder than the rest of the background, in the middle of the frame, the right hand side is not drawn. Congrats, you found the muzzle.
2. Follow the outlines of the darker areas with your eyes.
3. Or just use this https://imgur.com/a/dDpVtDa
Maybe the pictures in the article are just poor quality, but I can't really see what they are claiming to see. Even with the outline you gave. It just looks like a few lines that could kind of possibly sort of look like a deer head.
But yes, it's sometimes difficult and you have to have an eye for it.
One of the projects was to make reproductions of the crayolas used for cave painting. They mixed fat of ñandú (rhea, a local bird similar to ostrich) and iron oxide. The project was to use a spectrometer to measure how different fat, impurities and treatments changed the spectrum, probably to try to discover the original recipe and locals variations. (I don't have more details. Someone told me about this in a hallway conversation.)
Honestly, as soon as I saw the picture (having been to a few of the caves that are dated 30k+) it really is just immediately obvious that it would be included, there's a ton of examples that look almost exactly like it.