Read this [Script][Basics]Modding Beginner's Guide I - #2 by Elfe
Besides what’s written there (Asset Manager’s settings), I personally also like to cut & paste gdx1 extracted things on top of vanilla extracted things and then gdx2 on top that so that everything is conveniently in one folder and there’s no way of importing an older file if there’s an updated version of it in a dlc (since older files are overwritten).