This is where to begin! Adobe Photoshop Basics walks you through Photoshop step by step, with an emphasis on Astrophotography. Twenty-One Video Tutorials comprise this first series of IP4AP's Online Visual Learning.
Photoshop Foundations Intermediate Part 1 is the next step towards mastery of Photoshop for Astro-Image Processing.
Photoshop Foundations Intermediate Part 2 is the next step towards mastery of Photoshop for Astro-Image Processing.
Soup to Nuts
Soup to Nuts walks you through pre-processing theory, and practically applies it to MaxIm DL and Astroart. As an added bonus, it also provides an update on earlier releases of Photoshop CS.