Honestly I learned to sew mostly by myself, so here was my process:
I started out modifying clothes from Goodwill which helped me figure out garment construction a lot. After you learn how the different panels connect and learn how to visualize the seams, it becomes much much easier to look at a design and see what needs to be done.
I then transitioned to making cosplays that are within the realm of my skill (nothing that used anything complicated like darts or fancy seams). Things like simple unlined jackets or t-shirts are pretty easy to make. There are several things that will improve your sewing by a large margin, almost like a power jump. For instance, learning to use stabilizer and interfacing will improve your garments a lot. Or, getting a walking foot and stretch needles for knits.
The important thing is to break down cosplays into smaller parts, since every cosplay is just a combination of different garments. After you break it down, you can then look up tutorials to make similar clothing items, and modify it from there.
It is mandatory to make your prototype outfits out of shitty material in case you need to modify it for the final take. I recommend you use bedsheets, usually cheap at Goodwill if not free. You probably have some lying around.