You don't have to do anything. For your convenience, the bank that issued your credit card enables it for this service, without you having to change it or do anything else. Consult the banks that offer this service to their customers.
At any store that has the plazox service enabled. When paying with your personal credit card, the point-of-sale terminal screen (physical store) or the computer payment screen (e-commerce) will show the different installment options with their corresponding installments.
If you are shopping in a physical store you will only have to initiate the payment with your credit card enabled with the service, enter the PIN as in any usual purchase and follow the instructions. To do so, the store will give you the terminal.
Choose the number of installments you are interested in and complete the process following the instructions on the terminal. If you are shopping at an e-commerce and your card is enabled with the plazox service, the purchase process will open a screen with the installment options. You will be able to split your purchase in one click by choosing the split option you are most interested in and giving your consent by checking the acceptance of conditions button ("Do you want to defer?") that will appear below the split options. In both cases you have the option of not paying in installments and the operation will be carried out in the same way as in any purchase without installments.
Any type of purchase can be paid in installments as long as the amount is at least 60 euros. Merchants may, by prior agreement with their bank, raise the limit of this minimum amount up to 500 euros.
Purchases can be divided in 3, 6, 9 or 12 months.