On leather seats and door trim upholstery, gently work a detailing brush (or a soft toothbrush if you don’t have one) with cleaner in a circular motion, always applying cleaning solution directly on the brush instead of the seat to prevent over-wetting the seat or getting cleaner on spots you’re not working on. After scrubbing you can wipe it all off in one go with a microfibre cloth, then apply a leather conditioner.
With fabric seats, thoroughly vacuuming should do the trick. For stains, brush in some general purpose cleaner with a small amount of water. If your fabric seats need a proper clean to get stubborn stains out, use a wet vacuum. Try not to soak your seats, as trapped moisture can create mould.