So I took my UNO ICP schematic and converted it to ICSP connectors. This makes it easier to hook up, then connecting six jumpers to various header pins on the Arduinos. See schematic
There’s a couple of ways to do this. You could take 2 2x6 female headers and solder wires or you can take six female-female adapter jumpers like these.
I used the second method.
Two Arduinos with ICSP/ISP connectors
6 f-f adapters
1 m-m adapter
Identifying pin 1 on ICSP/ISP connectors.
These connectors are 2x3 male headers. See picture. Sometimes pin 1 is identified by a dot or a 1 or if you turn the Arduino over, pin 1 is square instead of round pad.
What I like to do is make sure by using a DMM and check which pin is connected to ground. This is pin 6 of the connector and pin 1 is the opposite. See pinout drawing.
For future reference, I mark pin 1 with red nail polish.
Now you have pin 1 identified on the two Arduinos. Identify one as the source Arduino. Source
is the Arduino used to bootload the other Arduino, the Target.
Take one of the f-f adaptors and connect 1 to 1 on the two Arduino ICSP/ISP connectors.
Take another and connect 2-2, 3-3, 4-4 and 6-6
Take another f-f and connect one end to pin 5 on the Target
On the other end, connect the m-m adapter, plug the other end into D10 female header of the Source
See picture and schematic.
Then I wrapped both connectors with transparent tape to keep them bundled in order.
And I marked each Pin 1 with red nail polish. See picture.