Positive to positive, negative to negative. Period. If you do it the other way, you will have an 18 volt dead short across two batteries and one of them will likely blow, spreading acid in all directions. It is not a good idea to jump 6 volts with 12 but if the starter is engaged harm will be held to a minimum.
When these questions come up, I can only assume that our school science programs have failed to teach the most elementary information about basic electrical circuits.
Good for you for seeking advice but I implore you, go back to my first sentence and under NO circumstances look directly at either battery when carrying out this very risky operation.