STEP 1 ----------------------------------
Open the following template: https://imgur.com/a/u1IXBSJ
The colors in it may differ a little from the in-game ones, but you can check the sprites in the stage files to correct them if you want. It shouldn't make much of a difference, though.

STEP 2 ----------------------------------
Draw over it, using only the colors in the corner of the picture. Please respect the outline color and the eye/mouth shape. You can distort the shape and size of the body but try to be consistent with the style. There's going to be examples in the end of this post so don't worry.
For the sprite on the right, just rotate the image and change the eyes and mouth. Make sure that the feet are aligned so the sprites are properly separated later.

Here's a helpful tip if your character is human/humanoid and has hair: if the hair doesn't go up for some reason like a messy hairstyle or a pompadour, the upper outline of the hair should be three pixels above the head of the template given above.
Kyoko's and Leon's sprites illustrate this rule pretty well, so you can use them as a guide.
https://imgur.com/a/QBDw46H

STEP 3 ----------------------------------
Delete the colors in the corner, you don't need them anymore.

STEP 4 ----------------------------------
Resize the image to 200%.

STEP 5 ----------------------------------
Save the sprite in the sprites folder of your character as XXXX_strip2, with XXXX being a name of your choice.

STEP 6 ----------------------------------
Put this in init.gml:

guiltySprite = sprite_get("XXXX")

And this in load.gml

sprite_change_offset("XXXX", 31, 0);

STEP 7 ----------------------------------
That's it!

Here are many examples:
https://imgur.com/a/eZo39cB
Those same sprites are in the files, so check them out for more accurate colors. Not that it matters.

I'll reiterate my advice to follow the template's eye shape for consistency's sake, which means that the eyes should be 2x2 squares if possible. It's not really necessary, but at least it adds to the immersion.

Another thing: Pay attention to the body size. See how, in the examples, bigger characters have bigger sprites

Anyway, have fun!