Pogo pogo pogo! For those times when you don't want a through-hole programming header interrupting your artwork, or taking up precious space on the other side of your PCB, and also don't want a chonky SMT header on your board. These have 0.3" spacing between the two columns to interface between with SMT footprints, and 0.1" spacing between rows. There are 2 versions: one for interfacing with a 6-pin IDC programming ribbon cable, and one for a 10-pin cable.
This will be either a 6-pin or 10-pin connector depending on the version you got. It is directional, so be sure to line up the notch in the connector with the notch lines in the silkscreen on the PCB. Solder the connector into one board - they're both the same, so it doesn't matter which one.
Assemble the boards and hardware
Using the provided standoffs and screws, screw the two boards together, making sure the notches line up on the same corner.
Solder the pogos
Drop the pogos through both boards, their smooth rounded ends should rest against the black connector. Make sure all pogos are resting at about the same height. Solder them to both boards, using the extended pads to get a nice fillet to each one. Flux helps solder flow to the pogos.