Login to https://developer.paypal.com/developer/applications.

Under "REST Apps", create a new app, called "involve_me" (or similar). Using these credentials involve.me will create orders in your PayPal account.

After the app has been created, you’ll be redirected to the Sandbox details of the app. In your involve.me account under https://app.involve.me/payment-accounts click the “Link Account” button under Test Payment Accounts and enter in the Sandbox details as a connection. 

To test a payment using the sandbox account, you need to login using a sandbox account. To find your sandbox login credentials navigate to SANDBOX > Accounts in your PayPal developer dashboard and toggle the account you want to use or create a new one. Using the link "Profile" below the account email you can see the "System Generated Password". Use this information to login during a test payment. 

To collect real payments you will need to add a Live account. To do this, get the live credentials from PayPal, by toggling to "Live" up the top right of the REST app details. Enter the details by clicking the "Link Account" button on https://app.involve.me/payment-accounts under Live Payment Accounts.

After you have connected your PayPal accounts, you can then use them to take payments in forms by drag & dropping the Collect Payments content element in the editor on the Thank You / Outcome pages of a specific project, selecting the specific PayPal account in the Other Payment Accounts dropdown.