When someone in the United States of America who once served in the armed forces dies, a military honor guard presents a folded American flag to the family at the graveside committal service. Many people buy or make a wooden three-cornered flag case with a glass front. Although a cheap flag case can be had for as little as $20 US, a decent case made of solid wood can cost over $100 US.
At this link you can read about the significance of the folds in the flag and other things related to the ceremony.
Step 1: Take dimensions
I chose to make a case for the flag that draped the coffin for my wife's father when he died a few years ago. He served in the US Army during World War II in the Philippines. The first step is to measure the folded flag. I am using a piece of newspaper to make a paper pattern. The flag is positioned with 3/4 inch of space around it for the wood that will make the case. Unfurled, the flag measures 5 feet by 9 1/2 feet. Folded, two sides are 17 5/8 inches. The third side is 25 5/16 inches. The folded flag is 3 1/4 inches thick. See the second photo. Your measurements may vary, depending on how tightly the flag is folded.