We can use von Baer's rule to assign polarity . Assuming von Baer's rule is followed, we see that the state of characters in Amphioxus is the primitive state. Connect the founder of the clade we are considering to the Amphioxus branch, not one of the other two. Your cladogram is complete, and, so we believe, correct. A is the outgroup or sister group.
Jaws appear within the vertebrates.
Legs evolved too within vertebrates, and did so after jaws, or so we believe. Legs define the Tetrapod clade. We begin as before. Notice something rather very important. The frog that we added to the first table fit in nicely, neatly, with respect to more primitive characters.