Adobe Education Exchange has a ton of free learning material. Of course, if you you aren't an Adobe user you will have to adapt the courses to your app. But I expect you could use this in Affinity, GIMP, etc. As long as it supports layers and masks.
This Guardians of the Galaxy Poster project is part of the "Basics Principles of Design" course. It's far from perfect and of you see the project image I have left out the actor names and some ships. I did add a custom color-grade and some texture to the bottom though.
Anyway, if you have decent chops and want to try out the project, I believe you only need an Adobe login to access the materials. It was fun and challenging.
The project says licensing is, "non-commercial attribution." Both of which I have satisfied. So I am pretty sure sharing here is fine.