If you don't want to learn to sew for this project, you might be able to just cut the curtains yourself, then use some heat-fusible seam tape and a hot clothes iron to seal a hem where you want it.
Thermal curtains usually have two layers of fabric, so you can tuck/fold both layers inward a few inches from the cut (so the outer surfaces now face each other at the very end). Place a strip of the magic tape (it's really just a hot melt glue coating a strip of fiber tape) the width of the curtain in the space between the inward-folded ends, press with the iron long enough to melt the glue, let cool, and you're done! It won't be as strong as a sewn hem, but it should hold up pretty well for a while.
You'll find the tape at fabric stores, but it goes by different names - I'd look for a heavy duty version for this use, but the regular stuff should still work.
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Thermal curtains usually have two layers of fabric, so you can tuck/fold both layers inward a few inches from the cut (so the outer surfaces now face each other at the very end). Place a strip of the magic tape (it's really just a hot melt glue coating a strip of fiber tape) the width of the curtain in the space between the inward-folded ends, press with the iron long enough to melt the glue, let cool, and you're done! It won't be as strong as a sewn hem, but it should hold up pretty well for a while.
You'll find the tape at fabric stores, but it goes by different names - I'd look for a heavy duty version for this use, but the regular stuff should still work.