When paying your invoice you can chose different payment methods. One of those are a scheduled payment, you identify yourself with Mobile BankID and choose the bank account you want to use.
Step 1 - Choose Pay
Step 2 - Choose Bank transfer
Step 3 - Choose the date the payment should be done
Step 4 - Choose the bank account that you want the payment to be made from. If you have made a schedule payment before your account will be saved and you can easily use that account. If it is the first time you do this payment you have to choose Add an account
Step 5 - choose Confirm payment