The bottom piece of flashing (painted gray) ran out over the face of the siding.
First, I marked the location of the next course of siding on the underlying course with a pencil line.
Then I installed a rectangular piece of aluminum flashing so that it covered the top edge of the siding and extended down to the pencil line.
Next, I cut a piece of step flashing at an angle
following the line marking the bottom of the next piece of siding.
I installed my first full piece of step flashing so that it extended down to the bottom of the starter course.
After nailing down the first course of shingles over the starter course, I slipped in the remaining step flashings.
I left a proper gap between the siding and the shingles ...
... so that water running down the step flashing would flow unobstructed off the roof.