Flag halyards are small halyards designed to raise and support a flag. The backstay is a great place to fly a flag because it will allow the flag to fly clear of any obstructions that could otherwise tangle or snag the flag as the wind shifts.
Now, to install a backstay flag halyard, you have to install a block of some sort high up on the backstay,. This will be the maximum height that the flag will be raised, and there are two options on how to do this. First, you can climb the mast (via the backstay) and attach the block on the backstay itself. The second method is to tie a hitch knot that will hold in one direction, and slide in the other.
I opted for the second method, as we do not need the flag to be too high up, so by tying a hitch knot on the dyneema backstay, I was able to then simply push the knot up the backstay with a pole. This allowed me to raise this point without leaving the deck! (Be sure to install the halyard in the block before you push it up the backstay though).
This hitch knot will slide up but when the block is pulled down, it will bind on the dyneema and hold firmly in place.
If you have a backstay adjuster, you want to make sure that the flag halyard remains lower than the lowest position of the backstay adjuster. This will keep the flag halyard from interfering with the more important function of the backstay adjuster.
With the halyard installed, you can fly your flag from the backstay!