I hope this is the right place to ask.
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There is a mod called 'Bella Goth (Sims 1)' created by ronyu02. They used Bella's model from The Sims 1 and is used as an in-game clothing.
It gave me an idea for a project and I would really like to know how it can be done.
I have The Sims 1 models and textures, all I need is a tutorial / guidance on what to do next.
Any help would be appreciated

UM, others will need to answer this, but my guess is this is not possible. The games were done totally differently.

It is possible, but as far as I know there is no tutorial for it. The process would be similar to converting clothing from the Sims 3 or 4: scaling and fitting the model to the Sims 2 body, copying said body's bone assignments to it, making sure everything is named and grouped properly before importing into the game. Obviously, this require general knowledge of how to create Sims 2 clothing meshes.

Tumblr won't load for me atm, so I can't link direct tutorials, but you can check https://sims2tutorials.tumblr.com/ for clothing conversion tutorials. I remember Rented-Space having a good one for 3to2 clothing.