It's rather simple to create the art but slightly more complicated to implement in, first you need to make the assets, this means you need to make the clothing for each pose Asterion has, he has 4 poses, neutral, hunched, front and surprise/rage (surprise and rage share the same pose). once that has been created you name the file in this format: *INSERT CLOTHING TYPE*_*INSERT POSE NAME*_*NAME OF CLOTHING*
for my flower crown mod I named my hat as "hat_neutral_flowers". when implementing it into the game (simplifying this big time btw) each update is done like a script, from top to bottom it shows the entire update and you have to add code in the middle of this script like code to give the player the ability to choose it in the wardrobe. so a custom clothing program would be rather awkward to sort out because there isn't just a single piece of code that handles inserting every piece of clothing into the players wardrobe at once