Simply set an amount that will be automatically debited from your bank account. This amount will be converted periodically (every week, every two weeks, or every month) into the amount of silver you can buy at the ccurrent 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 unfavorable time.
You can always view your accumulated amount of silver via your GoldRepublic account, where you can also easily adjust, pause, or stop your periodic contributions whenever you want.