Most people like a large, bright window that drenches the room with sunlight, but if a security floodlight points at the window, you need to catch daytime sleep or you just prefer dim rooms, the window is a nuisance instead of a godsend. Also, too much sunlight can also overheat a room, resulting in higher summer utility bills. Instead of throwing up the dreaded tinfoil, which many HOAs and lease agreements prohibit, make a set of inexpensive black out curtains.
Draw a line above the window to represent the rod you'll hang once you've made your blackout curtains; if you already have a rod installed, skip this step. Use a long level to ensure a straight, even line. Make sure the line you draw is several inches above the window's casing and extends equal lengths from each side of the window.
Measure the line you drew, and then multiply that measurement by 1½ and add 4 inches. Increasing your width measurement allows your curtains to better block light, gives them more fullness and allows for a 1-inch seam on each side of the two panels you'll create from this width measurement.
Measure from the line you drew to 1 foot or more below the windows' casing. Measure all the way to the floor if you want floor-length curtains, which will block light better than shorter curtains. Add 2 inches to this measurement for 1-inch seams on the panels' tops and bottoms.
Cut out four sections of fabric that match your final length and width measurements; cut two sections from your curtain fabric and two sections from your blackout fabric.
Lay out a regular fabric section, or panel, over your workspace. The patterned, or colored side, also known as the "good" side, should face towards you. Lay a blackout panel directly over the regular panel; match long sides to long sides and short sides to short. Place the blackout panel's good side facing down. If both sides look the same, just match up all the edges without worrying about which is the good side.
Mark your seams down and across each of the blackout panel's four edges. Use a straight edge to ensure even, exact markings. The seam lines should be 1-inch from each edge. Pin the two panels together along all four marked edges. Place pins outside of the seam lines you drew.
Sew the pinned panels together along your drawn lines, using a straight stitch. Remove straight pins as you get to them. Leave a 1-foot opening along one of the shorter edges. Repeat the marking, pinning and sewing process for the remaining two cut panels.
Reach inside the 1-foot hole in one panel and grab a handful of fabric. Pull it through the hole carefully and keep pulling more fabric through, to turn your curtain panel inside-out, exposing the "good" sides of the panel. Reach inside the panel with a pencil and poke at each corner to make crisper corners. Repeat the process with the remaining panel.
Tuck the edges of the open 1-foot section of each panel to the inside. Mimic the surrounding seams with your folding, and pin the material in place.
Transfer a panel to your ironing board. Iron the panel's edges flat, including the 1-foot section. Remove the pins from the 1-foot section, insert 1 foot of heat-fusible hem tape, and iron over the seam for 1 to 2 minutes, or as long as the tape's packaging directs. Repeat the process for the remaining panel to complete your lined blackout curtains.