It was actually a pretty easy install. Flip over your foot control then use a volt meter using the ground coming in from the plug and test the 2 wires on your foot on/off switch to find which lead is switched. It should read voltage coming out of that switch no matter the speed setting because it is before the speed control. Checking this first will help to ensure the mod will work for you and you won't drill a 1.5" hole in your foot control for no reason.
Once you've established it will work on your trolling motor unplug it then you will need to remove the speed control. For me the big knob you turn just pulled off. Then you will need to remove the nut (9/16 I think) holding the speed switch in and it should slide out of the way so you do not damage it accidentally while drilling the hole. Then use the plate that comes with the gauge to find where you want to position it mark center with a marker and I used a hole saw to drill mine then touched it up with a die grinder.
Now you're ready to mount the gauge I don't think the screws that came with the gauge were stainless so I swapped them for stainless ones. once you have it mounted you should be ready to wire it up. Remember which lead on the on/off switch was switching? Just use some small gauge wire and crimp a small ring terminal on 1 end and the supplied female spade on the other then just remove the screw on the switch you found earlier and put the ring terminal on it and push the spade connector on the gauge terminal marked with the + symbol.
For the ground just use another piece of small gauge wire then I just used a razor blade and removed some insulation from the big black wire coming into the control from the plug and wrapped the end of my ground wire around it soldered it in place then sealed it with some rubberized tape. Now all that's left is crimping on the other supplied spade connector and plugging it into the terminal with the - symbol.
It should be good to go!
On a side note the motorguide logo and frame around it on top of the foot control is raised I just used the die grinder I had out to remove the part of it I needed so I could mount the gauge flush but as you can see from the picture it doesn't look very nice. I would have used a razor blade to scrape off what I needed if I was to do it again so it would look nicer....
Also if you search for waterproof volt meter on eBay there are more sellers and even other colors of gauges like blue or green. I would have done a blue one since my boat is blue but none came with that bezel to hold the gauge and you will need that to mount the gauge in the foot control because the underside is not a flat surface.
Well have fun and good luck I can't wait for it to stop snowing so I can use it on the water!!!