First, flip up the metal tab on the rear hub as shown: (this metal tab IS the cable end hardware)
This video may help illustrate getting this one piece off....you only need to watch the first 15 seconds or so:
https://www.youtube.com/watch?v=DiG0dLox4qk
Then, get your 2mm allen wrench, and loosen the set screw just enough that you can slide the hardware AWAY from the cable housing (so that the hardware is closer to the end of the cable) by about 1/8". Later, you will be able to adjust out any excess slack using the barrels at the shifter.
Then, re-tighten the set screw (quite firmly so it doesn't slip), and reinstall the cable end hardware on the hub as shown in this video. Fast Forward to about 1:10
https://www.youtube.com/watch?v=hMNusK7SxSE
Getting the hardware OFF will be the hardest part (because it is so tight) it should go on quite a bit easier (but never as easy as they show in the video!)
Once it's all back together, take it for a ride. You can tighten the cables by unscrewing the barrels if needed-- you can feel "stops" every 1/4 turn. I would go 1/2 turn at a time and test.