Go to gocardless.co.uk (or .com) and register. Once you have done so, you will get a link which you give to your customers (text or email etc) or even to use on your website. This link takes them to a direct debit mandate to fill in with their bank details (below is mine).
Then basically once youve cleaned the property you process a payment either directly from your gocardless dashboard or via squeegee app etc.
It takes 3 working days for that to be processed with their bank and a further 2 working days with gocardless before you get paid. Within the 3 days the customer get an email telling them when it will be debited and whatever message you attach. For instance I put, "windows cleaned during the week starting mon X"
Its that simple.