Step-by-step guide on how he dun-did-that
1. Block out any creases or details around the but for a clean area to work with. Images with more detail on the underwear or gradients should be replicated afterwards.
2. Try to replicate outlines or textures to get as close to the original style as possible.
3. Add the bulge as just an outline and adjust for perspective. In the third image I cut out the underwear and used a mesh-transform to make it bulge more naturally. I had to clean up the blurry lines afterwards, tho.
4. Add your fecal matter. This artist uses flat shading on almost everything expect for areas that needed definition, like the legs, so I used a gentle gradient on the fecal matter to make it pop better.
5. added extra fecal matter around the scene for effect, using black outlines and gentle gradients to match the original style.
6. Lock the transparencies pixels on the colored-over underwear layer and use a soft brush with a color just below white and leaning in the orange range to create a subtle stained color on the underwear.
7. Do any other FINE adjustments you need