Forming a cladogram from a table of information
I think the clue is at each step to look for the trait that is shared by most of the remaining taxa.

In your example: 9 out of 10 taxa are vertebrae. So you are correct to pick this trait first and outgroup Lancelets.

Looking at the remaining 9 taxa:
8 have jaws, and a different set of 8 swim bladders or lungs
At this point you can pick one of those traits, as far as I know.
You chose jaws and outgrouped Lampreys.

In the remaining 8 taxa, the characteristic that is shared by most of them (7), is having swim bladders or lungs. Thus, you can outgroup Sharks here.

In that way, you can go on for the remaining traits and taxa.