Go to https://app.involve.me/payment-accounts, choose between Test Payment Accounts and Live Payment Accounts, click on the Link Account button and in the dropdown select Braintree. Follow the prompts to connect an existing account or create a new Braintree account.

You can use the account that is not in live mode to test payments.

After you have connected your Braintree 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.

From the Collect Payments element, select one-time payment, then select the appropriate Payment Account.