Battery 9V DC Jack 2PCS
This power jack adapter is for the rectangular 9V batteries. It also has a 5.5 by 2.1mm center-positive DC jack, which connects to MCUs with on-board female DC barrel jacks like Arduino Uno, Mega, Due, and the likes. The wire is around 10cm long.
It is straightforward to use. You need to attach the snap-on part of the adapter to the 9V battery. You won't get confused as the snap-on has 2 different parts (Female/Male) that snap right on the battery.
You could also use this to connect your DIY electronics to the battery. This pack comes in pairs.