I've got a bathroom window (older house) that wasn't done right. There were no real attempts made to seal it. Most importantly, there's no flashing around it, and I was told that would need to be done. How would I install flashing without removing the window? Could I possibly slip it in there under and around the window?

I'm assuming it needs to go in between the window and the studs and bend around the outside siding. Could I just nail it flat all around the window and seal with silicone or something?

I Need To Know What Type Of Exterior You Have Brick, Viynl, Ect,,

Originally stucco, but the area around this window has all been removed, and it's now just a piece of plywood! Like I said, it wasn't done right. It's the very back of the house and no one will ever see it, so I can do whatever kind of exterior is easiest there.

If the window has a nailing flange you can use flashing tape, you'll tape the bottom first, then the 2sides and then the top, making sure all the joints lap over each other.
If you use metal flashing flash it the same way as the tape