First you need to remove the old window and the framing molding and the metal frame that is attached to the 2 x 4 framing inside the walls. This is a flange that you nail or screw to the surrounding 2x4 framing.
You really need to open up the entire window framing area.
Once the raw opening is exposed, you will see the 2x4 framing around the opening. Now, the flanges of the new window should fit over the 2x4's on all four sides. If not, then you need to reduce the hole to fit the window or expand the hold to fit the window. Reducing is easier than expanding, but I hope that your window will fit as it is if your measuring was correct.
Now, screw the flanges in all around. Now, do any caulking or weather proofing. Place everything back as it was, and as you go, do any additional caulking as you go.