I used tailors chalk and marked the center back line. I used my painters tape and placed it along the outline of half the back piece. Then I covered the whole thing in painters tape.
Because it fell apart the first time I decided to reinforce the painters tape with duct tape. The duct tape is crosswise to the painters tape.
Ta Da! They don't lie perfectly flat and the center back and center front lines are not exactly straight. But it's a good start. I will make small corrections when I trace and prepare them for seam allowances.
Happy Victoria Day weekend!