The first step should always be a discussion with your line manager as to what you want and need, and why. Ideally the line manager should then discuss what they feel would be acceptable.
This discussion should provide a better idea of what would be accepted to make the process easier. If this does not provide a positive outcome, you can still apply as ultimately it will trigger the below process which could lead to it being approved or still refused as detailed below.
A flexible working arrangement must be submitted in writing and submitted to your line manager for consideration.
The request must state:
It is helpful if any request gives as much information as possible about current work patterns, the flexible working changes requested, and the reasons why.