To register for time Out-of-Programme with both the RCoA and HEE, there are forms to fill and here are examples of forms that have been submitted before, along with an example personal statement which went in with this.
If you want to extend the OOP period beyond the previously awarded period.
Original form available here: https://www.rcoa.ac.uk/system/files/TRG-OOPTRApplication2016.pdf