The GoCardless for Salesforce Billing app is available directly from the AppExchange here.
Before you can install the app; the following prerequisites must be met:
Multiple Currency Mode
The org on which you are installing must have Multiple Currency mode enabled. Please be aware that this process is not reversible. For more information on Multiple Currency mode, see Salesforce's help article here.
To enable Multiple Currency mode, go to Setup for your Org and type 'Company' into the Quick Find bar. Click 'Company Information' then click 'Edit'. Scroll to the Currency Settings and click 'Activate Multiple Currencies'. For more information on Multiple Currencies, see Salesforce's help article here.
Quotes
Quotes must be enabled for your org. To enable Quotes, go to Setup for your Org and type 'Quote' into the Quick Find bar. Click Quote Settings and then click enable. For more information on enabling Quotes, see Salesforce's help article here.