This pic has a lot more going on than the last one. The red arrow again shows the groove for where the screws go.
The double green lines, is the space between window frame, and the frame of the wall. There needs to be a little clearance there to allow the window to be slid into place. Yeah, that was a long time ago.
When I glued the insulation to the wall, it extended past the frame of the window. The area between the 2 blue lines was insulation and needed to be cut off.
I figured it was easier to extend the insulation a little bit and then cut the excess off.
The yellow line is where I put the loooong utility (box cutter) blade, and just cut through the insulation right next to the wall frame. Easy as pie....
And that, is how and why I bought the box cutter. It worked very well to remove the excess insulation, and leave the insulation flush with the side wall.
If you want to do something, you will find a way.
If you don't, you will find an excuse.
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Good judgement comes from experience.
A lot of experience, comes from bad judgement.