Now to connect it up. You need to do the plumbing
before cutting the liner. At some point I want to do some hard PVC, but at this point the Intex lines are fine with me. One thing at a time...
The 40 mm to 1 1/2" adapter I linked above works fine for this. The Hayward skimmer has inside and outside threads. The adapter screws into the inside ones.
There's an o-ring seal, so you don't need thread tape, even thought he adapter kit comes with it.

One note though, if you screw it down too tight, the o-ring will bulge out. If this happens, unscrew until the o-ring goes back, then re-tighten to a point before the o-ring starts bulging out.
The 59" Intex hose I got previously worked fine, but unlike the gaskets the Intex hoses use to connect to the pump that fit nicely inside a hole, these hoses came with flat gaskets. With no centering portion, it was a bit annoying to get it to seal up, but after a few tries I got it. It seemed to help to put the gasket in, then before attaching the hose, slide up the gray threaded ring as far as it would go, then make the connection while holding the threaded ring up.
At this point it now looked like this. The original dual inlet was disconnected from the pump, and my 59" hose went from the skimmer to the pump.
At this point I got myself a container, a towel, and a beer.
I used the container to fill the skimmer up to the top with water to check for leaks.
I used the towel to dry off the outside of the skimmer, the pool, and the hose.
With everything dry, I let it sit for about 5 minutes to check for leaks.
This is important to do before you cut the liner!
Turns out I had a leak at the Intex hose at the bottom of the adapter. I disconnected, re-centered things and got it re-connected. Re-filled with water, then dried everything off again and waited another 5 minutes or so. Nothing leaked this time.
Now that it was verified to be leak-free, it was cutting time. I took the razor blade and carefully cut out the liner inside the skimmer opening. I rounded the corners a bit...engineer me doesn't like square corners, but it's probably fine in this case...
Mask and snorkel came in handy again here, though again not strictly needed.
Added the weir door.
Added the skimmer basket.
And it was go time. I opened the valves and turned on the pump!
The only thing left to do was put the skimmer lid on, and we were done.
At this point I got my Intex auto cleaner running again (because of the leaves seen in an earlier picture), poured myself another homebrew...
...and there we have it! One skimmer install, on an Intex pool, with no draining.
Couple extra thoughts:
Some things may be different, if you already have hard plumbing, or use a non-Hayward skimmer. Adjust appropriately.
Note that I have gone from the Intex dual inlets to a single inlet on the skimmer. This probably isn't the best. The dual returns are due to the
Virginia Graeme Baker Pool and Spa Safety Act, which requires that pool drains be "unblockable". With one inlet, if the inlet is covered up completely there will be a tremendous amount of suction force, and people, especially children, can and have died by becoming stuck underwater on inlets. In the unfortunate case of Virginia Graeme Baker, a 7 year old girl who the law was named after, she became stuck on a hot tub inlet. Her mother was unable to pull her off. Two men eventually were able to pull her off the inlet, but the suction force was so great the inlet broke in the process, and it was too late for the girl.
I say this to point out the risk involved in modifying plumbing to go from multiple to single inlets. With two inlets spaced appropriately apart, the suction force never becomes as extreme, as water can flow through the second, unblocked, inlet, and people are unlikely to become stuck underwater. I plan on adding a second skimmer at another location in the pool. Either that, or I'll reconnect one of the original inlets so I have one through-wall inlet and one skimmer. But right now I don't have the plumbing to do this, so for the moment this is how it is, and I will use appropriate caution (if we had kids, I wouldn't have left it for any length of time like this).
All that said, a skimmer is not the same as a regular wall inlet or floor drain, as there is a hole in the lid. Even if the opening is blocked completely, the pump can pull air in through the hole in the lid, keeping the opening from pulling a truly terrifying amount of suction. Still through...
Finally, while everything went smoothly for me, as always there is some amount of risk with this procedure. I take no responsibility for any actions taken as a result of this post. It's likely easier, and perhaps a bit safer, to drain the pool at least to a point below the bottom of the proposed skimmer.
In any case, I hope this is useful for some people. As I said, I've seen a lot of people add these, but never a lot of pictures of the process or details, so I hope this makes the process easier for people in the future!