The two half hitches is a knot which tightens around poles and hooks. It is often used to tie guy lines to dining flies, for dragging logs, and for boating. This knot is considered "not adjustable" because it tightens around the object that it is tied around when the end of the rope is pulled on.
You will need a rope and a pole (e.g. a tent stake, or a pvc pipe) to tie around. It will be easiest if your pole is horizontal since the pole in the demonstration is horizontal.
Place the rope so that it is looped over the pole.
Then loop one end of the rope around the circumference of the rope on the other side of the pole.
Now pull the end of the rope below the loop and loop it around the circumference of the rope again.
Now pull on the end of the rope to tighten and complete the knot.
Once you master the two half hitches, try the taut line hitch.