I don't have any reference for that garment, although I am familiar with Hatoko's work.
I'm not saying what you're proposing is impossible, since it involves working with an existing model (which simplifies 35% of the work). This means taking the garment and modifying it in Blender. However, it's important to verify if duplicating the mesh of one leg of the legging also copies the weights for the bones of that leg, and to test the pose mode to look for loose or unanchored bones. Skipping any of these details, which could cause a critical problem, will prevent the leggings from breaking in the game.
Within Unity, it's somewhat more "routine" due to creating prefabs, placing materials, 2D textures, the mesh (or meshes if there are multiple garments), the thumbnail, and the asset bundle itself for the prefab and thumbnail. Although this is where I get a little bogged down, and why I don't make clothes for Koikatsu, is working with the clothing script, physics, and configuring the bone list with the CSV file. This takes me a lot of time and frustrates me even more when I need to log into the game to test if it works or not.
That's why I really appreciate zipmods that include clothing, and if they have any bugs, I do a little digging in SB3Utility before diving headfirst into Unity to tinker with things.
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