If you want to set up a direct debit, you are probably best doing this by calling O2. All numbers are in this guide Guide: Coronavirus Community Help and Support
Or you can complete the direct debit mandate here. Print it off, complete it and send to the address on the form http://static.o2.co.uk/assets2/pdf/O2_DirectDebitMandate.pdf
Veritas Numquam Perit
Prior to edit it was a question regarding timing.
The payment is due 14 days after the bill is generated.
First bill generated on 9th Sept and payment taken on 23rd if I remember correctly.