My skylights needed to be 46.5" wide, and 46.5-50" long (they were supposed to bow a little bit). Since I was altering curtains, half the work was already done for me. I just needed to cut the curtains down on two sides to make them the correct size, and then add a curtain rod sleeve to the bottom end.
First, I cut the curtain shorter on the right side. The curtain was now 48" wide. I also unpicked approximately 2 inches of the curtain rod sleeve (at the top right), so that I could finish the new sleeve edge neatly.
I stitched the side seam down at 1/4".
As I have mentioned before, in my towel hoodie tutorial, with large projects I prefer to roll up the project for easier maneuvering. Then I feed the project into the sewing machine slowly, with my right hand guiding the un-sewed portion in front of the sewing machine, and my left hand guiding the already-sewed portion from behind the machine. I sew slowly, and adjust the fabric very couple inches. In this way, the weight of the project does not pull my stitches all over the place.
The side seam. Again, verifying width with each pin:
Once both side seams had been stitched, I again pinned and sewed the bottom hem, to create a clean edge and then create the second rod sleeve:
The skylights are recessed into the ceiling. These extending shower rods will be used to hold the covers in place:
The finished skylights: