It's summer, and that means people are getting their vans road ready for the fall climbing season! One of our additions has been blackout window coverings for those times you need to stealth camp, that also didn't look like we pasted tin foil in our windows.
Fit it in the window and use a sharpie to mark how much to trim. 75% of my time was spent here getting a perfect fit without cutting too much. It's like that tale of the boy who wants to make a perfect heart for his valentine, makes a hundred cuts, and ends up with a minuscule size heart. Anyways, take your time and don't cut too much.
I missed a photo of the next step, but you want to fold the very end of the right side over ~1/4" and iron it flat to make a nice edge (rather than a raw edge), then lay the left side on top of it, then stitch the whole thing down to finish, like this tutorial "Joining Two Ends" section.
[Sidenote: You could put the bias tape around the insulation and sew once to catch the top and bottom, but in practice it's nearly impossible to be this close to the edge with a straight seam and catch both sides since you can't see the bottom.]