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. 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.