Today we are going to do an exercise.
1 - Install GIMP https://www.gimp.org/
2 - Chose an image pair you like. If you don´t have any at hand, there are many here: https://booru.allthefallen.moe/posts?tags=boppin
3 - Make sure you click on "View Original"
4 - I will use https://booru.allthefallen.moe/posts/607109 and https://booru.allthefallen.moe/posts/607110 for my example.
5 - Download the image pair
6 - Open GIMP
7 - Click File -> Open as Layers
8 - Select the two files
9 - Click Open
10 - If you don´t see the layer window, use CTRL-L to open it
11 - Drag the image with clothes so it is the top layer, if needed
12 - Right-click on the layer with clothes, then on Duplicate layer
13 - Click on the eyes on the left of the layers to make only the copy and the naked version visible.
14 - Click on the eye on the left of the copied layer multiple times to confirm the differences between the two images
15 - Count the number of separate clothes
16 - In my example image, there are three pieces: left sock, right sock, tie.
#loli

17 - You should have at least two pieces. If you only have one piece, chose a different image pair and start again
18 - Duplicate the copied layer multiple times until you have one copy for each piece of clothes, and rename the layer with the name of the piece.
19 - In my example, the layers are "left sock, right sock, tie".
20 - Select the top layer, make it visible using the eye, and make the rest of the copies invisible
21 - Click on "Free Select Tool"
22 - Click near the desired piece of clothing, and click around it until you select around it, making sure you don´t select any parts of the other pieces of cloth.
23 - CTRL-I to invert the selection
24 - CTRL-X to delete
#loli

Next steps:
25, 26: https://gameliberty.club/@exlurker/112415516322414249
27: https://gameliberty.club/@exlurker/112415520641857079
28: https://gameliberty.club/@exlurker/112415526867025217

29 - Rename the clothes layers to add a number 1 to the top layer: example: left sock 1
30 - Duplicate the number for each clothes layer below: right sock 2, tie 4
31 - Duplicate the number again to get the total number of possible images. Example: 8
32 - The images will be named 0, 1, 2, ... until one less than the number of images you can generate (example: 0, 1, 2, 3, 4, 5, 6, 7)
32 - Make sure that you enabled the top layer (1) and the background (naked) layers, and export the image to disk, using the number as filename.
33 - The original naked image will be image 0
34 - The original clothed image will have a name which is one less than the total number you can generate (example: 7)
35 - You can use binary numbers (https://en.wikipedia.org/wiki/Binary_number) to decide if a layer should be visible in a specific image. Example: Image 5 has layers 1 and 4; 1+4 = 5.