Sock tutorial:
Search for pictures with feet and without socks.
Example
https://booru.allthefallen.moe/posts?tags=feet+-socks&z=5
Choose a picture where the lower leg can be selected easily and it's not obstructed
Example
https://booru.allthefallen.moe/posts/927626?q=feet+-socks
Open the image in gimp
Separate the lower leg into layers
Duplicate Layer
Add alpha channel
Use:
-Free selection
-Diffuse selection, antialiasing, feather edges, threshold 30, radius 1
to remove the unneeded parts of the image (images 1-6)
-Move the selection so one border matches the new sock border, and put the pivot there, then rotate so the line goes along the mark (image 10)
-Repeat until the new border is complete (image 11)
-Rotations may smooth the border, darken it as needed with the paint brush (images 12 and 13)
-Copy and paste to fill the sock gaps as needed (image 14)
-Smooth as needed, but keep the texture
-Remove shadows as needed using copy and paste again (image 15)
-Select the bottom curve of her shin to use as the new top of the sock. (image 16)
-Remove the top of the leg, which should not be covered by sock (image 17)