First: learn what you can about form and lighting! Draw, sculpt, paint -- study environments and photographs. An understanding of these principles undergirds all successful manips! (Note: this includes playing with focus, grain, color variation, irregularity -- all things needed for pieces to look photorealistic!)

Second: learn the tools in your software of choice and don't be afraid to experiment! Years ago, I did a tutorial, and really I don't have many techniques besides the ones I outline in it. https://www.furaffinity.net/view/16459802/

Third: do stuff you're excited to do. Do bad manips. Your first manips will be bad: that's perfect. Keep going!