If you elect to take a period OOPP you must discuss and agree this with your educational supervisor and training programme director. This agreement is to ensure that going OOPP does not destabilise the educational or service sufficiency of the programme you are in. After this, the application can proceed once details of when and where you will be spending the time out of programme have been confirmed. The OOPP section of the OOP form should be completed, together with the OOPP initiation document that sets out your plan for OOPP, your intended scope of practice, indications of possible capabilities you may acquire, plans to return to training and any pertinent indemnity considerations.
You can find the OOP forms and guidance here