Set the header in place, and tack it there
by soldering one of the pins on the top
side of the board.
Then, on the bottom side,
solder all six pins in place.
It goes in location J2 on the circuit board,
oriented so as to match the drawing on
the circuit board.
Part
#15
is a six-pin single-inline
(SIL) right-angle (RA) header.
Assembly Step 18 – Six pin SIL RA Header
[19]
[Alpha Clock Five Assembly Guide]