Once you have your GoCardless account activated and have added your client’s Bank Account payment details, you can then setup recurring bank payments for your clients.
Head to your 'Clients' tab, then select client
Select the 'Finances' tab
On the 'Recurring Payments' section, click on '+ New Recurring Payment':

Fill in the details of how the recurring payment should be setup:

Note: the payments will be taken on the first day of each period displayed.
Related articles:
How to process a once-off GoCardless payment from my client's Bank Account?
How to setup a recurring GoCardless payment linked to a Membership
How to view, change, skip or cancel GoCardless recurring bank payments