In order to process customer payments, your online store will need to use at least one payment gateway to connect to payment processors such as Paypal, Stripe or Authorize.net. The payment processor is responsible for collecting payments from your customers, notifying you payments' status and depositing the money to your designated account upon requested. The first step of accepting online payment is to establish an account with one of those payment processors. After that, you may configure the respective payment gateway to start processing payments.

Your payment processors should provide you with an API key/user and a token/password to use with their payment gateways, some even provide a test account for you to test their gateway before actually going live with the real account. The test account works exactly the same as the real account except that no actual money is processed. SiteGUI Appstore has many Payment Gateway apps, choose gateways you have an account and add them to your Site. After that, head to Apps listing to activate and configure those gateways with your API credentials, you may enter the provided test credentials and enable Test mode to test the payment process. A Display Name should be defined for the gateway, this will be shown to your customers when they choose a payment method. Finally, set the gateway to Active status to start accepting payments through that gateway or inactivate it if you do not want to show it to your customers. Some gateways support payment notification Webhook/Callback to notify payments that are processed later. In that case you may enable Webhook/Callback with any code to get a webhook URL and give it to your payment processor to setup the webhook.