Often, I find that the flour that sticks to the dumplings is enough to thicken my broth sufficiently. If I want it a bit thicker, I add a slurry of water and flour, a little at a time, until I like the thickness. Make a slurry by whisking together flour and enough cold water or broth so it's pourable. Do not make a slurry with hot liquid or your flour will clump up.