Log in to your PayPal account
Hover on your top-right menu,
Then choose Account Settings
Under API access click Update
Scroll down to NVP/SOAP API Integration (Classic)
Click Manage API credentials
Choose the Request an API signature
Click Agree and Submit button
Then it will show you your API signature page
Click show to show your API username, API password, and Signature.
Log in to your WordPress Dashboard
Navigate to WooCommerce -> Settings
Click on Payments on the top menu
Click the Manage button on PayPal Standard
Key in the username, password, and the API signature you get from your PayPal account dashboard.