Very simple really, you only have to extract the assets of the game (for example with unren), convert them to PNG and then in an editing program such as Photoshop you can assemble them as if they were a figure.
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Something like this:
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It can be turned into something like this:
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You can also improve the scenes with the improved designs that are in the game files but are not implemented yet, so you can convert this:
Original scene
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In this:
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Basically that's it, but of course, there are people who already do a job beyond simply edit assets but create them from scratch, in other words, own creations based on the characters of the game, there are a few around here, those personally are the ones I like because the others that are created with assets is something that I can do, drawing, on the other hand, I am not good at