... “Second problem: You’re underage, which means you’ve still got the Trace on you.”
“I don’t —”
“The Trace, the Trace!” said Mad-Eye impatiently. “The charm that detects magical activity around under-seventeens, the way the Ministry finds out about underage magic! If you, or anyone around you, casts a spell to get you out of here, Thicknesse is going to know about it, and so will the Death Eaters." ...
Use polyjuice potion to disguise Harry and Mad-Eye Moody as Dursleys. Drive away in the family car. Go somewhere nearby that is connected to the floo network or that has a portkey. Transport Harry to a safe house from there. Have other order members disguised as the same Dursleys drive the car back to Privet Drive.