I have a flashed XBOX that recently decided to update it's firmware to 13599. It now won't play any games.

I'm assuming I need to flash it with a custom firmware to allow it to work again, but don't have a clue what to do.

I knew I should have learned how to do this myself instead of asking someone else to sort it originally. Any help welcomed

you will need to connect xbox to pc via a via sata card of 360usb
then flash it using jungle flasher and the latest fw for your drive
loads of tuts around on how to do it

Jungle Flasher - Downloads here is a start

I'll give it a go. Thanks

Would a X360USB PRO do the trick?

The usb pro allows data connection between PC and DVD drive.

You will still need to be able to power it. CK3 mini is cheapest proper way.