So, if you just want to overwrite an existing outfit, what you need to do is add (inside the game folder) an images folder, and within that a LauraSprite folder. Since the files linked in that post are already named to replace Laura's jacket, you can just copy them there. You might also want to add some blank pngs named Laura_Sprite_Jacket_Back.png and Laura_Sprite_Jacket_Back_up.png, to replace some outfit bits that are not present with the new outfit piece. This will result in Laura's jacket looking like the shown Mavis top when looking at her normal standing sprite, but will do nothing for her sex poses (as the relevant sprites are missing).
If you want to try adding the top as a new piece, you'll need rename the parts and then create modified code that adds a new outfit option and references those new names. That gets a bit more complicated, though not necessarily difficult depending on familiarity with coding. The relevant script file you would need to change can be found in my
earlier post, though the ones there have obviously been modified to add the outfit parts seen in that post. If you wish for unmodified ones, you'll need to find something to decompile the archive.rpa. To find parts that would need to be added, I'd suggest searching through those files for anything referring to Laura's jacket (probably just called Jacket in most of the code), and making a copy that points to the Mavis top by whatever name you chose. Though she does start with the Jacket by default, so you'll either end up with her automatically having the new top, or you'll need to tweak things so you need to buy it (as well as making it so you can buy it).