What is the process to train an exotic (aerial) creature to be a mount?

During my current campaign we came across some knights riding aerial beasts (Giant vultures and Hippogriffs). We sack their tower (killing off all the knights/beast trainers) and found giant vultures and hippogriffs in their pens. They have already been trained for mounting but are not friendly to us.
What needs to be done/how long will it take to have them used as mounts for us?

Please use RAW in your answers (as our DM may not agree with other DM rulings), designer comments are ok.