Hi Nicolas,
Are you installing on a Doepfer LMK2+ yourself? If yes, the TouchKeys installation itself is fairly typical, following the video instructions, but there are a few tricks with the case.
Removing the keybed from the Doepfer is perhaps the hardest part. You need to remove the control panel at the left, then undo all the large screws in the base of the wooden case. Do the last couple from underneath with the keyboard sitting on the edge of a desk, because if the keyboard is turned vertically the keybed will drop when you remove the last screws. You then have to carefully lift the key bed over the lip at the front of the case and pull it out. It's awkward but fortunately not too sensitive.
Once you do that I recommend putting little bits of tape at the back of the key levers to hold the cables in place. On some LMK2+ keyboards, the white keys have a little plastic spike near the back left over from the injection moulding process. In the worst case if you press a cable against this you can puncture it, so filing these down is a good idea.
Test the TouchKeys installation before you try to put the key bed back into the case. I usually cut a hole for the USB port toward the bass end of the keyboard, near the existing ports (e.g. just above that panel). You can buy a USB extension coupler (USB B to USB A) and glue that into the case. Then internally, you can run the mini-USB cable that comes with the TouchKeys kit, taping it into the back corner of the case.
Finally, when you put the key bed back into the case, make sure the TouchKeys controller boards don't get caught underneath. You might find it helpful to tape them to the back metal part of the key bed. Once they're inside they should be pretty robust. And of course, don't forget to reconnect the connectors for the key bed and the aftertouch strip!
Best wishes,