I am in the process of this manual install and I too had difficulty visualizing everything. I'll try to get some pics posted. In the meantime, there are at least two options I know of:
1. Cut the light blue wire from the glow plug controller under the dash and take it to one terminal of your momentary switch, bypassing the glow plug controller all together. The other end of the light blue wire is already connected to the glow plug relay on the firewall under the hood. Connect a new wire to the second terminal on your switch and connect to a ground. When the key is on and you hit the switch, you will be manually controlling your glow plugs.
2. Remove the light blue wire from the glow plug relay under the hood, tape off, and replace with a new wire. Run the new wire through the firewall to one terminal of your switch. Connect a new wire to the second terminal on your switch and connect to a ground. When the key is on and you hit the switch, you will be manually controlling your glow plugs.
Option 1 is easiest/quickest. Option 2 keeps the glow plug controller (wiring harness) in stock form for later reuse (if needed).