Building extension jambs on windows

We have decided to build a 2x4 wall in front of the exterior walls of our home we are remodeling. Everything seems easy enough but I’m getting caught up a little on framing the windows out in the interior wall. All the windows are vinyl replacement windows. I removed all the existing trim so the window is sitting in the rough opening, with approx. 3/8”-1/2” gap on 3 of 4 sides(bottoms are sitting directly on bottom plate)
I envisioned that I would just use pieces of 1x to extend the gap between outside wall and inside of our new wall. I know the exterior walls are out of plumb a little bit so I’m not sure what my best course of action is to make sure these new jambs look good. First window I used a level to get the new framing in line with the window, but that really took a while for me and I didn’t feel like that was the right way to do it, hence, my questions to you professionals.