Noob questions:

Have some upcoming work flights which I would like to upgrade to F, but haven't yet booked them.

How do I actually use a Joker... do I have to have a booking first?
Must the booking be direct with BA or can I apply to a ta booking as well?
Finally, I would be able to choose whether to use a GUF1/2 or avios to do the actual upgrade, right?

thanks in advance!