Natural light is actually great for training your eye, because essentially, what you see is what you get, and you can focus all your attention on camera operation and composition. If you’re trying to get detail in makeup, you’ll want to diffuse (scrim) the light to assure that you’re lighting evenly and you’re not getting harsh shadows (unless that’s specifically the look you’re going for). Again, it’s easy to make adjustments because you can see it with your eyes before you even take the photo. You’ll learn all the essentials of lighting that can be easily transferable to a studio environment.
In terms of retouching, I’m not really sure what to tell you. It’s important to learn how to retouch skin without making look in any way altered, but ultimately, the goal is to develop good taste. If you have a clear vision of how you want your photo to look from the onset, then there are infinite ways of accomplishing the same end, so your taste will dictate how good you are. Everything else can be learned via tutorials, probably.