I still consider myself inexperienced, but iam getting there. Its alot of experiencing how things work out if you use them and then you gotta experiment with it.
Heres some things i use the most in photoshop:
1. Image > Adjust > Match color and also color lookup.
2. Using layers and blend options.
3. Using a brush that has lower opacity and flow so it looks like its smooth and doesnt produce harder lines. The spot healing brush then can bring both together, thou it doesnt always work.
4. Burn tool and highlights obviously espaciley for walls.
5. Different layers for different shadows, and also make use of the blur tool to make stuff stand out by putting some field of depth onto the image.
Also this thread on reddit is a good way to start out: https://www.reddit.com/r/battlemaps/...e_tutorial_v2/ espaciley the big pack of items in it.