You must be logged in to My Pages. There, select “My Västtrafikkorts” and choose the card you want to auto top-up by clicking the button to the right. You will see a short description of the service and can confirm your order by clicking the "Order now" button. You will then be taken to the shopping basket and should follow the instructions given there.
If you have Auto top-up set up for your card, when you balance falls to, or below, SEK 0, your card will be topped up by SEK 200. If you buy a new top-up from a point of sale or at a Västtrafik shop, it can take up to 48 hours before you are able to auto top-up via the website.