Basically it’s solvent. Will be either water or oil based. In most cases if stain is internal and quick drying. It’ll be water based. That’s the one I would go with. If external wood stain or no mention of quick drying on the front of the tin then it might be oil based. So you’ll need white spirit or turpentine.
The big question is how much to add. For modelling you can really thin it down to nothing more than a wash. But just test it as you go until you get what you want.