This article explains how to manage your GoCardless mandates.
Go to the relevant customer
Click on the tab "Payment Methods:
In the right column "Bank" you can see the mandates:
In order to add a mandate, click on "Add payment method" -> "Bank account"
Send the link provided to your customers. You can also use smart fields to generate a unique link automatically to everyone of your customers. Please refer to the smart fields help article.