Enable Module
Enable the PayPal module for example Off-Site Payment configuration.
Payment Methods in Drupal Commerce are installed as contributed modules. Most contributed modules add configuration screens to Products, the Store Configuration, or other similar pages in the administration side of your Drupal installation. Payment methods simply enable a rule that shows up in the Payment Methods found in the Store Configuration screen.
So how do you configure these payment methods? You might think you can simply click on the payment, add your details and click save. Most administration screens in Drupal operate that way. In order to enable further functionality, we have opted to simply make the payment methods full rules. That means if you want the PayPal payment to show up on checkout, you need to go to this screen and click "enable" and then click "edit" for the rule, then click "edit" for the action that stores all of your developer credentials.
Below we show how to enable PayPal from beginning to end. There are other kinds of payment methods and those are described in further detail at On-Site vs. Off-Site Payment Methods.
Enable Module
Enable the PayPal module for example Off-Site Payment configuration.
Enable Payment Methods
Once you've enabled the modules, they will appear in the Payment Methods section of the Store Configuration. A lot of people will go here first before enabling the modules.
Enable Method Rule
Before you can configure, let's go ahead and enable the off-site payment method example PayPal
Configure Payment Method
After you have enabled the rule, you can now click "edit" to configure your payment method.
Configure Payment Method
Once you click "Edit" you are presented with the rule configuration screen. To edit your credentials/etc you need to click "edit" on the actions. You could also add conditions to when this payment method should be used (only on orders over $50, perhaps?).
Off-Site Payment
Off-Site Payment Method is now functional for PayPal.