We deduct a fixed monthly amount from your stipend, before tax is calculated. This is paid at the end of each month to an approved Payroll Giving Agency and they will distribute it on your behalf to your chosen charities. The agency will charge you an admin fee to cover their costs, further details can be found on their website: The Charities Aid Foundation (CAF)
You can either choose to donate to one charity or split the donation between multiple charities each month. You can also build up money into a personal account with CAF and manage your donations from there. You can only donate to registered or approved charities (this can include PCCs). The payroll giving agency will check if a charity qualifies.
Agencies are required by law to forward donations within 60 days but the agency will be able to give you more information about their own policy.