With this smack-your-forehead, it’s-so-obvious, single-flag configuration, there’s no measuring the exact distance between where the clips will be applied to accommodate flags with varying spaces between their grommets. It’s a simplistic approach that works well in light conditions for shorter periods of time.
Make sure the halyard line will be the right length, and tie a small carabiner onto each end. Before raising the pole, tie on a third carabiner to the top and clip the middle of the halyard through it. The clips on each end of the halyard attach to each of the two grommets of the flag forming one continuous loop with the flag itself joining each end of the halyard.
There’s one, important point to remember! The halyard by itself does not form a continuous loop unless the clips are linked together. This means, when the flag is not attached, if one end of the line happens to get pulled upward past arms reach, there’s very little likelihood it can be retrieved without lowering the whole pole. So, after the pole is raised, whenever the flag is not attached, always remember and be careful to join the two clips together!
SCOUT PIONEERING 
Equally as simple yet with more teaching opportunities would be to place the lower clip on a small prusik tied to the halyard. This address stress concerns for the flag and also accommodates flying additional flags on the staff.
This is an update on the traditional rope and toggle rigging. Very uncommon on US Flags..
I wonder about additional stress on the seam and grommets of the flag when raising and lowering, as well as the flag having to carry the added weight of the portion of the halyard beneath the flag when it is fully raised. Over time that might shorten the life of the flag.
Quite right. I’ve added a qualifying sentence to the opening statement.