I'd highly recommend removing the drivers seat so it's easier to get under the dash. I built the gauge harness outside of the boat and then installed it. Removing your old gauges first will make measuring easier.
If you are running new power to the dash, I'd recommend removing the old wires from the harness. The harness connectors separate to allow removal of the pins. Expect to get covered in black tape residue if you do this. IIRC I had to clean the old harness with goofoff as I went. I added a new breaker for my dash run next to the battery switch, as the original factory fuse holder is too small. I also added bus bars on the back wall of the dash to easily connect to the new runs.
When you cut your harness at the dash to splice in the new gauge harness make sure you leave yourself enough of a tail after the connector. A bad crimp with a tail that is too short can ruin your day.
Download a marine wiring color chart. Our boats followed the marine wiring color code. That will help you get the right harness wire connected to the correct gauge.