While you're in your Organization, select Processors from the left-hand menu, then click on Add Processor.
Select the processor you would like to add among Stripe, NMI, Checkout, Airwallex, Mastercard Gateway, Sandbox Processor and Network Ngenius.
A pop up menu will appear, requesting you to fill out the needed information to add your processor.
Once done click on Save changes.
Once you are done connecting your processors, you will need to create a Payment Flow for your store.