Step 1: Apply to register a paypal merchant number on the paypal official website.
https://www.paypal.com/signin
Step 2: Create the APP you need on the developer management page of paypal. Most companies will create a REST API application. Similar to Alipay H5 payment.
https://developer.paypal.com/developer/accounts?event=linkAccountAssociated
Step 3: Download the latest paypal code from paypal open source on github.
https://github.com/paypal/
Step 4: Get the paypal source code l Press Make First REST Call to connect, and replace it with your own client id and Secret.
Step 5: HTTPS and TLS 1.2 security mechanisms are required to communicate with the paypal server since June 2017. So jdk voluntarily upgraded to version 1.8 or later, and applied for an https ssl certificate for its own domain name. (If you only have a free 3-month trial version online for research and study, you can apply for one)
Step 6: After the fifth step is completed, you can use the simulator in the developer management page of paypal to test your webhooks interface.
Time is short, the above steps are only general implementation methods. For details, you can check PayPal's docking documentation and online help.