Measure the dimensions of the glass panel to be covered. If you are replacing existing curtains, measure the space from the top of the top curtain rod to the bottom one if you are using double rods.
Purchase curtain rods if the window does not already have them. Select thin rods, up to 1/2-inch in width, to fit inside the headings you will sew. Mark where they will be mounted then measure that space to determine the length of your curtains.
Add seam allowances to your initial measurements. If you are using double rods, that is, rods at the top and bottom of the window, allow 1-1/2 inches for a top seam and the same for the bottom. For example, if your window/curtain rod space measures 36 inches high, add 3 inches for a total of 39 inches of fabric. If you are using a curtain rod only at the top of the panel, allow 1-1/2 inches for the top heading and 5-1/2 inches for the bottom hem -- 4 inches with a 1 1/2-inch turnover. This deep hem gives your curtain a little more weight to keep it from blowing every time the door is opened. For this curtain example, you will need 41-1/2 inches of fabric: 36 + 5-1/2 inches.
Calculate the width of your fabric. For a single curtain, whether hung on one or two rods, allow 1-1/2 times the width of the panel for some gathering in the curtain. If your panel is 20 inches wide, you will need 35/36-inch-wide fabric. For either one or two curtains on a door with a central glass panel, you could use 48/50-inch-wide fabric for a slightly lusher look.
Look for fabrics specifically designed for sheer curtains; voile, made of cotton or synthetics, micro-fibers and nylon or other net fabrics work well. Some retailers gather them all together in a category called "sheers." Avoid silk fabrics because they deteriorate in sunlight. If in doubt, a synthetic or synthetic blend fabric might last longer than a natural fabric when exposed to constant sunlight and dirt blown in when the door opens. Synthetics, however, may not be washable with bleach. Ask your retailer about fabric care.
Lay the cut fabric face-down on a work surface. Fold and pin the top headings: make a 1/2-inch-deep fold then a 1-inch fold. You may want to iron the folds rather than relying on pins alone. Use a sewing machine or needle and thread to stitch the folded heading.
Make the same headings for the bottom of the curtains held by the top and bottom rods. Fold, pin and iron them. For curtains on just a top rod, fold, pin and iron the deep bottom hem.
Check the curtain dimensions on the mounted rods. Make any minor adjustments to the bottom headings/hems. Machine-stitch or hand-sew the bottom headings/hems then enjoy your new curtains.