The hardest part of the process is probably the woodworking associated with making the new companionway drop board(s). Here is a link to a search with tons of info to get you going down that path:
I'll share a few details/thoughts from my experience that may help but can't create a detailed how to since all companionways are different, design needs may vary and AC units will vary as well.
First off, I'm very happy with my setup so can recomment it.
An important design consideration as you plan how to do this is how low the threshold/bottom of your companionway entrance is. My boat is a coastal sailor with a low step into the cabin
so adding the AC (as low as I could get it) means that I just step over it as I enter/exit and it's not too big of a deal. On some companionways this may not work that well so you will need to get the AC as low as possible and may want to add a step or make it easy to get the AC in/out of the cway repeatedly. That may necessitate a different and more complicated arrangement than what I went with but it would be worth it on some boats and may even be a safety
Another detail to consider is adding a handle to the AC to make it easy to carry and install/remove. I made a rope
handle (drilled holes and ran rope
from one side to the other over the top) and slid a piece of aluminum
pipe (light and large/comfy diameter) over the rope. It's setup so that the handle stays at the balance point of the ac in both directions so that by just holding the AC by the handle it is level and ready to drop into the companionway. Without a handle or with a handle that tilts the AC you may fight to get it lined up and dropped into the slots. I can easily carry my AC around, etc. thanks to the handle and wouldn't be without it.
I did add a bit of foam weatherstripping when I installed the AC into the hole in the replacement lower drop board that it fits into. You need to install it in a way that slopes the AC a little down/away from it's front and with that in mind you should be able to get the AC in the drop board in a way that may not leak without a seal. The seal helps insulate and keeps the water
out in extremes. Think about how these things sit in windows with minimal sealing but where the outside is a bit lower so rain/condensation is encouraged away from the inside.
The AC that I installed had a nice bolt on flange that was really useful in mounting my AC in the drop board hole. I ended up bending the lower tabs and drilling/screwing them to the dropboard on the bottom and then using the bolt-on flange up top and screwing it to the drop board. That means that the AC is really locked into the drop board and doesn't shift at all as it's carried around or set down/etc. I doubt all of the AC units could so easily be adapted and screwed in but you'll have to figure something out to get the AC mounted securely. I'm pretty sure that I ended up flipping the top-flange around backwards/etc. to get the AC angled correctly. You'll have to take into account whatever angle your companionway boards have and then figure out how to get the AC itself slightly tilted outward and securely mounted. Fortunately you can cut the sides of the hole at an angle if needed, add wooden bracing/etc. to the drop-board etc. to get it right.
You may be able to get by with only making one replacement drop board (if your current
bottom board is large enough for the AC hole) but I had to make two and then I use my topmost board with the AC and when I have the non AC boards in.
My AC unit has a remote
control which I love since I can control things from the v-berth.. Spend the extra few bucks for one if you can.
I used cheap plywood
and since I needed 1" thickness I glued two .5" sheets
together. I then used the current
boards to trace the cuts on the new wood and that made it a quick/easy bit of woodworking. I used a hand plane to do final fitting of the new drop boards since the bottom isn't a consistent cut all the way around. I put lots of pencil (chalk would work or similar) inside the slot where the boards drop in and then would install the board and bring it out and then could see where to plane since the high spots picked up the pencil markings. There are probably better ways to get the fit nice and tight for the boards but it was a fun process for me and didn't take too long.
That's all I can think of for now. It's not too hard and on lots of boats is probably the best/easiest way to add AC and this time of year on the East Coast
will be well worth the effort. Good luck getting it setup.