You can usually get away with using as much product as you want. Don't cover the pad completely with product or anything, but drawing a circle about half of the diameter of the pad is usually a good way to go. There really isn't a 'too much' or 'not enough' when it comes to applying sealants. If you used more than necessary, then you just get to spread it around on more panels of the car. Just draw a circle on the pad that is about half the size of the pad and you will have enough to start out with. Once you get used to using the product and the machine then you will understand a little better about how far the product will spread and how much to use. :bigups
Oh, and use as soft of a pad as you can. No need to use anything abrasive. Just use a soft foam pad.