Just mount the yagi on a metal mast and run a clamped on wire from
the bottom of the mast down to a rod driven into the ground. If you
want to use a wooden mast then just clamp a wire to the boom or
the mount of the antenna and run the wire perpendicular to the boom
and away from the antenna down the wooden mast. The ground driven
rod is ideally eight foot long but it is unlikely you'll be able to
use the full length. If possible drive the ground rod close to
grounded plumbing like your outside water spigot. Attach a short
jumper to the plumbing also.