The problem is, there is no set process. It really depends on the source material. There are plenty of photoshop tutorials on YouTube for how to use specific tools.
I would say familiarize yourself with the following.
removing a background from an image.
Using the spot heal and patch tools.
Resizing specific parts of an image.
Clone stamp.
Changing the brightness//tone/contrast.
Text effects.
Layer styles - especially glow, drop shadow and emboss.
Understanding these tools will make it much easier to do simple covers, and it will give you much more of an understanding of how photoshop works and what can be done with it. Just don't try and learn everything at once - take one subject, and watch 3 or 4 videos on it, and play with it. The results can differ depending on the source material, and how you approach it. The only way to learn is to dive in and play.