I've installed, and re-installed several Raymarine tiller autohelms in boats the past couple years. One BIG mistake I've seen from previous owner installations is trying to install the autohelms as cheap as possible. This has led to some half-ass butcher jobs that ultimately didn't work. Keep in mind any of the autopilots need some type of adapter. They can't include the parts that may be required for every installation. I like to keep things simple, clean, and with minimal hole drilling. The two critical installation points is the autohelm needs to be level, and at 90 degrees to the tiller when centered. Figure out what adapters you will need to correctly and neatly install the autohelm, and have them with you when you begin the project, (and keep in mind, the parts required for a neat and working installation are probably not found at Home Depot or Lowes). Also think how the installation will workout down the road, i.e., if you install an adapter on your tiller, will the tiller cover still fit on? Will the installation still allow you to use the cockpit cushions? Think about what electrical device or electrical run in the autohelm's location that could cause problems with the internal fluxgate compass. If you need to move the autohelm location an inch or so forward or back to get a better, easier installation, it won't hurt a thing, but remember, this effects the range of motion, and the force on the tiller. Using my C-22 installation as an example, it's located slightly different of the recommended distance from the rudder piviting point. It just simplified my installation and allowed me to install the power plug in the cockpit locker with the autohelm just clearing the locker hatch. I hate drilling holes in my boat, and I didn't care to drill a BIG hole required for the autopilot power plug. So as the photo shows, it's located in the aft locker, and my Dremel tool made a nice easy point in the hatch to run the cable. This has worked out very well, is waterproof, and is also used to route my shore power cord when the boat is in the slip,(again, no big hole was required for the power cord hook-up). The cord moves back and forth freely without binding, and after several years and a lot of use, there is no damage or chaffing of the cord.
Good luck, an autohelm is one of the BEST upgrades we've made to out boat, and really makes getting to your destination less tiring from having to man the tiller all the time.
Don