You simply set an amount that is automatically collected from your bank account. This amount is periodically (weekly, fortnightly or monthly) converted into the quantity of silver you can buy for it, at the current silver price.
By periodically buying silver, you spread your purchase moments over time, also known as dollar cost averaging (DCA). Sometimes you'll buy silver when the price is high, and at other times when it's lower. This reduces the risk of buying a large quantity of silver at an unfavourable moment.
You can always keep track of your accumulated silver holding via your GoldRepublic account, where you can also easily adjust, pause or stop your periodic contribution whenever you like.