Simply set an amount that will be automatically debited from your bank account. This amount will be converted periodically (every week, fortnight or month) into the amount of silver you can buy at the current silver price.
By buying silver periodically, you spread your purchases over time, also known as dollar cost averaging (DCA). Sometimes you buy silver when the price is high, other times when the price is lower. This reduces the risk of purchasing a large amount of silver at an unfavourable moment.
You can always view your accumulated amount of silver via your GoldRepublic account, where you can also easily adjust, pause or stop your periodic deposits whenever you want.