It´s very easy to store your bag. You simply insert your debit or credit card, choose a locker, choose your own security number, accept the payment amount (see info on prices here) and then enter your card pin number on the credit-card machine. You can also use a contactless payment option (if your credit/debit card supports that) or by using Apple pay. Simply then bring your card or device to the credit card machine when the amount is displayed on the screen. Notes and coins are not accepted.
If it sounds complicated, don´t worry, the luggage locker will guide you through it. It´s simpler than using an ATM.

The luggage lockers are accessed with a pin code, chosen by the person leaving the bag in the lockers.
Anyone with that pin code and the locker number can retrieve the luggage so be careful that no-one sees the pin code you choose.

The luggage lockers have a touch-screen except for the ones in the Reykjavik Domestic Airport and in Laugardalslaug swimming pool.