Being able to ollie to manual up curbs helps with this trick. Ride to the box like you would for a 50-50, except go at less of an angle. Ollie up and don't fully level out. Land on the back truck and stiffen your leg so your back truck will lock into 5-0 (don't worry about tail trag yet). Don't lean back cause you will slip out. Keep your leg straight and hold it intil you are at the end of the box. Come off like a 50-50, this is easy since you are already doing a manual. Practice these for a few days and you should have them down. When you get good at these start doing them without tail drag.