I'm amazed at how much of an improvement you've gone through with your drawings! I made a really quick progress guide on my latest drawing for someone on the forums. It's not very good, but it's partly because I draw fast with minimal sketching. I skip out on more detailed sketching because I have a generally good idea on what goes where on a pony. However, when I first started, sketching took more time than lineart, because I sketched out every part and detail. It looked messy, but it made the lineart so much easier to draw. There's also some tricks to shading on SAI. If you need any more help just ask! I use Paint Tool SAI, but it should be similar to how gimp works.