I suggest you to use laundry detergent because it's designed to clean fabric right.
If you plan to use powder form, make sure you mix is properly. For ease of use, you can use liquid detergent like the new Surf excel that comes in liquid form.
If you have a stain on the seat, don't over saturate with the cleaning solution. Spray a small amount of cleaning solution and try to absorb it as much as possible with the towel by pressing it downwards. If you spray lots of cleaner and rub it with your towel, the stain will spread on a larger area.
Once the stain is gone or became light, then you can rub your towel back n forth as much as you want.