You could choose an image you like to decensor, if that helps, I just figured supplying one would help avoid the issue I often have of being too indecisive and just searching forever.
When I want to practice, I use decensoring. I've done one Uno Makoto decensor, but attempted several others as a way of try to learn complex shading. While I still haven't got the hang of it, it does make doing simple things feel easier.
Here's one I've currently been working on. Not super difficult, mostly flat colours, but obviously a lot of work to be done. I'll add smegma when I'm done, I'm not certain the style. The method I do is:
1 Cover bar censors in high contrast colour
2 Make layer above and use as clipping layer (IDK if Gimp had that. It would be using the censor covering as a mask), and add the base colours (whatever the most dominant colour is)
3 Add layer above again (masked/clipping) then draw the details atop the base colours
4 Add more layers for the details, eg highlights, an top of the colours (or if you're lazy you can just use the one)
5 Add final layer to be the line work (you could do this first if you'd prefer. When decensoring the dorse doujin (https://e-hentai.org/?f_search=Uploader%3Aconcor) I found it easier to start with the lines, but with coloured stuff I prefer to start with the colours)