If you want your workshop character to join OMORI and friends, follow these steps:

Step 1: Prepare a template
You should be editing a 147x45 image, which is 3 frames.
It's best to start by working off of a base. The most useful is probably Aubrey, Kel, or one of the templates. This one has hair with the same details as Omori and friends: https://imgur.com/a/e3Dd7Hd, this one lacks hair: https://imgur.com/VzTsTil.
If you wish to use a different character as a base, you can find all them in stage's folder within the 383980 folder. You'll find them in 2859177822/sprites/articles folder. If you use these, don't forget to size them down to half size before you start editing.

Step 2: Prepare your pallet
Each character uses 4 shades, 3 of one color, 1 of another. Most characters use the 1 shade as skin tone, and the other 3 as the outline, details, and facial features.
Most characters use a blue for details and an off-white for skin details with the following hex values: #12004a, #2901a2, #3106FF, #dec8Fe. These colors are present in the templates.
However, you can use a different pallet if you feel it makes your portrait look better. OMORI, Clarien, Sylvanos, and Wrastor are all examples of characters who use different colors. You can color pick from your character sprites, use colors from other OMORI art, or just use colors that seem right.

Step 3: Draw your neutral expression
Your character must include a neutral expression. Happy, sad, angry, and toast are optional.
It's best to start with a neutral expression. If your character is humanoid, the outline, shading, and basic features will be generally similar to OMORI and friends, as the source game has very similar features between characters.
Neutral expressions will have a small smile or a flat mouth, depending on what fits your character's personality. Omori, Kel, and Pomme are good references for your options for your neutral expressions
After you have your first frame, you should copy it and make slight changes to the outline and shading for the next two frames. The face will not change between frames.
After you create your neutral expression, see how it looks in-game (don't forget to double size it).

Step 4: Draw other expressions
After you have your neutral expression, you can draw on top of it to make your other expressions.

Happy sprites will have a smile or smirk. Eyes might be closed or narrowed. Hero, Aubrey, and Zetterburn are good references

Sad sprites will have a frown, squiggly mouth, or flat mouth. Eyebrows should be turned upwards. Eyes are usually looking down or off to the side. Kel, Aubrey, and Absa are good references.

Angry sprites will have a frown, open mouth like yelling, or sometimes a puffy cheek. Eyes may be narrowed. Eyebrows will be longer to make them glare. Hero, Kel, and Olympia are good references

Most characters use the default toast. However, if your character has some kind of facial accessory or other reference, you can make a toast that includes it. Your accessory should stay in the same position as compared to the toast. Keep in mind that the toast moves side to side and up to down. The toast does not count against your color pallet limit.
Aubrey, Mollo, and Clairen all feature their facial accessories. Shovel Knight's feature the horns on his helmet, and Zetterburn's is burnt.

Step 5: Resize
After your edits, you should resize to 200%.

Step 6: Variable definition
In your character's init.gml you can include the following variables:
omoriface_n = sprite_get("[your neutral sprite name]");
omoriface_h = sprite_get("[your happy sprite name]");
omoriface_s = sprite_get("[your sad sprite name]");
omoriface_a = sprite_get("[your angry sprite name]");
omoriface_t = sprite_get("[your toast sprite name]");

You MUST include an omoriface_n. If your other emotions are not defined, they will instead use the neutral expression. If you do not define omoriface_t, it will use the normal toast. Alternatively, omoriface_t can be defined with a string "aubrey" will give you Aubrey's toast, which features her bow. "zet" will give you Zetterburn's burnt toast. If you would like to use a different character's toast, you can simply copy it into your character's sprites.