Can someone help with having trouble understanding how to collect a payment for an invoice with go cardless after setting up a direct debit mandate?

Hello there, @philcleave!

You can start collecting payments from your customers via Direct Debit once their bank already processed the mandate you've sent to them. You can read through this article to learn more: Learn more about GoCardless Direct Debit for QuickBooks.

Lastly, here's a few articles you can read for ideas to help manage your customers and sales transactions: Manage Customers and Income for QuickBooks Online.

Keep me posted in the comments if you have any other questions. I'll be here to lend a hand.