Step 7: Test
Here's the test to make sure the secondaries are both pushing the same direction.
Our two secondaries in series produce 38volts AC with no load. That seems about right.
If they'd phased wrong it could have been fixed by reversing the wiring to any winding.
Where Tim says "out of phase" in the video, he means "in phase". That is, the center tap should be less than the outer two leads, and if things weren't that way, the transformers would be fighting each other, or phased wrong.