First you need to install Braintree Payment Plugin from your WebAsyst Installer, It will automatically copy the plugin files to correct folder on your webasite.
Now login to your webasite's administrator panel (backend).
Just Navigate as per flow - Shop -> Settings -> Payment -> Add Payment Option -> Braintree.
Here you are on settings page.
Check 'Enabled' Option.
Default values are assigned to 'Payment option name', 'Logo' and 'Description' fields, You may edit them if you willing to do so.
Put values of 'Merchant ID', 'Public Key', 'Private Key' and 'CSE Key' which you will get from your Braintree account.
If you want to Save Customer and Cards to Vault - opt 'Yes'
Select the currency in which you want to process the card.