The short answer is that he didn't; total war refers to the total mobilization of a society's resources for war, which would be above his pay grade, and second, the intentional destruction of all sources of enemy strength.

Sherman did not accomplish the latter objective. When Georgia governor Howell Cobb surveyed the state following the march to the sea, he concluded that no serious damage had been inflicted. Sherman did not destroy any of the major industrial facilities in Macon, Augusta, or Columbus GA, and almost inexplicably missed a chance to destroy the Augusta powder mill that supplied 90% of the Confederacy's gunpowder. No major railroads were taken out; after Hood's failed Nashville campaign, the battered remnants of the Army of Tennessee were able to get on trains and be transported from Tupelo Mississippi to North Carolina. Even military targets along the strategic line to Savannah were to be spared as long as the civilians in the area did not impede the march, and when they did, discretion was left to the corps commanders.

To me, what makes most sense was that Sherman was primarily interested in supplying his army; after John Bell Hood abandoned Atlanta, he set about attacking Sherman's supply line up towards Dalton, Chattanooga, and Nashville. Rather than following the First Commandment (Thou shalt fall back along thy line of supply), Sherman supplied his army off of the area of operations, which is just a fancy way of saying taking the food from the Georgia countryside instead of the railroad. Once he got to Savannah, the port facilities there could form a new base of supply and operations, as the Union had command of the sea by then.