I'm adding a wood double hung window to my house, and I ordered it
without casing so that I could try to match the casing on the existing
windows very closely. Now I need to install the window, and all the
discussions I have read assume your window has casings. So should I
add the casings first, or install the window and then add the casings?
Can anyone point me to to instructions for installing a window without
casings? My main concern is how to do the exterior flashing and trim
properly.

Here's some more details on my situation: the rough opening in the
stud-construction wall is framed and the interior wallboard is up, but
I haven't cut through the 1x10 sheathing or the shingle siding. I
understand that I can secure the casingless window by pinning it with
screws through the jambs to the trimmer studs. Obviously I will need
to remove the shingles where the casing/exterior trim/sill horns will
be. Should I expect to remove additional shingles for the flashing?

On a related note, I plan to reside and resheath my house this year or
next, so does that consideration affect any of my choices now?

Thanks, Wayne