A Half-Windsor knot is a bit smaller than a full Windsor, but it's still pretty darned big. Normal-sized people shouldn't use Half-Windsor knots except maybe as a joke.

To tie a Half-Windsor knot, drape your tie around your neck, with the fat end falling to the right. Wind the fat end half-way around the thin end, then bring it up and wind it half-way around itself. Now the fat end of the tie should be backwards on your right side. Take the fat end, wrap it around the front of the knot and bring it up, behind the knot through the "V" the tie makes at the base of your throat. Run the fat end through the loop in front of the knot and tighten.

See also Windsor knot and (my fav) the Four-in-hand knot.