If there are instructions on how to install, then I suggest you get the structure to the point that the instructions start at. Maybe not the optimal path, but at least you can install the windows as the manufacturer recommends.

Frame normally, leave an appropriate gap for window size.

Waterproof opening.

Put outside wall covering on. If flat that's great. If not flat you may want to allow for a flat portion the same width as the replacement window border so it can sit flat against wall.

Put window in with quality exterior caulk along the frame that hits the outside wall.

If the outside wall covering is some type of boards/siding , I'd see if it you can get it to just lay proud of the window frame for more water proofing.

I think you can still use them. You would probably prep the rough opening the same way as you would a new construction window, with a sloped sill (using cedar siding) and flashing installed in the correct orientation and locations. The important part will be how to you water proof it without a flange, but that will depend on how the window is constructed. If you wanted to get fancy you could probably even make a flange from aluminum stock and a good caulk like OSI quad max or something.

Just frame to appropriate RO and install them per manufacturer spec

>each window will take about four times as long to install.

I don't see why. New construction windows still need leveling, fastening, sealing and trim.

It is not ideal, but replacement windows can be installed properly in new construction, just follow the installation instructions.

You just put the screws through the sides of the window frame , inside the house , instead of using the nailing flange .

The screws will be visible , so get some that are as close to the same color of the frame as possible , unless you’re going to paint the frames .

We ordered our with the frames in the color we wanted so no painting required .

Somewhat related, but can someone explain the difference between the two?

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