How dark is your bedroom at night? Lots of studies show that the darker it is, the better you sleep. So if your curtains are too thin to block much light, here's a tutorial for you!
I choose black ripstop nylon for my liners because it really blocks light and it's lightweight. (Just make sure your curtains are heavy enough that you can't see the black through them.) I then lined the liners with muslin because I didn't want it to look, from the outside, as though I have black curtains!
To get the width of your liner, measure the distance on your curtains between the side hem stitching lines. (So not quite the full width of the curtain.) To get the length, measure from the bottom of the rod pocket to about half-way down the bottom hem. Add 1/2 inch seam allowance on all sides. This is how much ripstop nylon you'll need. Now add 4 inches to the width and 4 inches to the length. This is how much muslin you'll need. I was making two curtain liners but I got them both out of really wide muslin.
Cut a 4 inch strip off the length of the muslin and pin it (right sides together if it matters) to the nylon 3 1/2 inches from the bottom. Sew at the 1/2 mark. (You're sewing 4 inches from the bottom of the nylon.)
Flip the muslin over and press. If you've sewn a little inacurately and it doesn't quite line up, just trim off the longer piece, whether it's the nylon or the muslin. You want them the same length.
Pin the large piece of muslin (which should now be the same length but 4 inches wider) to the nylon, right sides together.
Stitch 1/2 inch from the edge.
Now stitch 1/2 in from your first line of stitching.
Take the muslin and pinch it over on the edges to create a pleat of about 1/2 inch. Do this on both sides. What you want is to take up that extra 4 inches of width so that the muslin is the same width as the nylon. Adjust the pleats until the muslin is nice and flat across the width of the liner. Pin in place and stitch the bottom of the liner with 1/2 seam allowance.
Do the same with the top of the liner as well but leave a gap of about 5-6 inches for turning.
Turn the liner right side out.
Press. The nylon side of the liner will have about a 3 1/2 inch band of muslin at the bottom and about 1 inch on the sides.
At the top of the liner, the muslin and nylon are even. At the gap you left for turning, press under 1/2 inch. You'll be stitching over this in a minute so you don't need to sew it closed.
Find the center of the curtain and the center of the liner by folding each in half and pinning. Line up the centers and pin the liner, nylon side to the wrong side of the curtain, just under the rod pocket. Work your way out to the sides, pinning the liner in place. If the widths aren't exact, try easing in the fullness as best you can, so that the liner comes just to the edge of the side hem. It's your choice whether to pin from the liner side or from the front of the curtain but it's much more accurate to sew from the front. (I first pinned from the back, to make sure the liner was right up next to the pocket edge. Then I moved the pins to the front.)
Turn the curtain over and sew about 1/4 inch lower than the rod pocket stitching line. Make sure you use the right color top and bobbin threads. If you're using two different colors, like I did, make sure the tension is correct on your machine, so the thread doesn't peek through.
From the back of the curtain, you'll have just caught the top of the liner, just below the stitching line for the rod pocket.
Hang up the curtain and tack the liner in place, about half way down the sides and at the bottom corners.
Enjoy your nice dark curtains!