Start the process in good time if personal factors make the implementation date of changed arrangements, assuming they are approved, a critical factor.
Submit your request as soon as possible and certainly not less than 12 weeks before you want the new arrangements to start. If your request is approved, your manager may need to put in place other changes locally and will need time to do that. There may be occasions where an unexpected change to your personal circumstances makes this length of notice impractical and your manager will do their best to respond rapidly.
If you decide to make an application, talk to your manager informally in the first instance. That will enable you both to think through some of the implications and consider whether you need to seek out further information before making a formal request.
To make a formal request you will need to complete a flexible working request form. It is important that you complete all sections of the form so that your manager and Head of Department (HoD) have the information they need to give proper consideration to your request. They may come back to you for more information and if so you will need to re-submit your form with the information included. The date of your request will then be the date of re-submission.
You may make a request jointly with a colleague and in that case each of you should complete a flexible working request form.