>>46952
A drawing tablet helps enormously.
If you want to use GIMP with a tablet, find the latest development release (v2.9) so you can have lossless canvas rotation.
Take care to match the lighting, line thickness, color palette, etc. in the original image.
Use lots and lots of references from your favorite diaper artists and look at them regularly.
Other than that, just keep practicing. Making good edits requires many of the same skills as making good original art, so any art practice is good.