Hi kmc,
Private Residence Relief is availabe for the period of time the property was your main residence, over the period you owned the property.
If Private Resident Relief can cover your gain, there is not Capital Gains Tax to pay and nothing to declare.
If there is tax to pay, you need to report and pay the Capital Gains Tax within 60 days of the completion date.
Please have a look at the guidance here:
HS283 Private Residence Relief (2024)
There is a Capital Gains Tax calculator here:
Capital Gains Tax
To help you work out your gain and also a link to register for a capital gains account to report and pay your Capital Gains Tax liability.
Thank you.