It’s not mentioned in the federal flag code, but by tradition if you have a flagpole that is too short for the flag to be lowered to half-staff (such as a home flag mounted diagonally off the porch or window facing) then it is respectful to signify honor and mourning by flying a black streamer from the top of the flag pole. It should not be attached to the flag, but to the pole.
Here is an easy way to make a streamer. Use a length of ribbon that is twice as long as of the flag, and the same width as one of the stripes. Take an elastic hair band (like you’d use for a pony tail), and knot the ribbon around the hair band so that it looks neat and tidy. Trim the ends to the same length, then dress them by notching in swallow-tails or by cutting them on an angle. Twist the hair band around the top of the flag pole, adjust until it hangs freely and looks nice.
For flags mounted on a wall or hanging by the header attached to a pole, you can place streamers at the top corners, but on the wall or pole, not to the flag itself.
A caveat: Decorating ribbon is less expensive, but will fade on your flag if it gets wet. Dressmaker’s ribbon (for use on clothing) is more expensive but will last longer and is colorfast.