We purchased 2 of those small hooks that attach to standing rigging. Attach one 1/3d of the way up the backstay and the other so you can reach it from the deck. Put a small block on each of the hooks (they have a hole in them) and a continuous tight line. Put the flag on with a couple of flags clips and hoist it up. Same as a flag halyard at the mast.

That's where ours lives. Works great.