Hi Bob,
Login to GoCardless, click "+ Add Customer" and then click the "From Link" tab on the left hand side of the window that appears.
Your "Direct Debit Authorisation" link will then appear. That's the link you want to email to customers and/or link to from your website.
I should add that the Cleaner Planner GoCardless Widget handles all this for you. Simply drop the widget onto your website and it adds a "Pay with GoCardless" button and crucially a "Reference" box (missing from the standard GoCardless form) where the customer must enter a reference (e.g. first line of address) before proceeding to sign up and payment. This makes it much easier to figure out which address each GoCardless sign up relates to. Without it all you get is name and email which might not be enough e.g. if you don't keep track of customer names or if someone else at the address signs up to GoCardless and you don't recognise the name.
Thanks,
Rob