Find a large area in which to practice shifting and maneuvers on your bike. A large parking lot or multi-use path is ideal.
Lift the rear wheel of the bike off the ground, or set the bike upside down on its seat and handlebars. While spinning the pedals of the bike clockwise, pull the right lever to its topmost position and push the left one to its lowermost position. You are engaging both shifters into the "one" position.
Set over the bike and begin riding at a decent pace in a straight line. Going too slow will result in jumpy, erratic shifts, so work up a good amount of speed.
Take your right hand off of the handlebar when you feel you are pedaling too fast to keep up with the speed of the bicycle. In one quick motion, hold the bicycle steady with your left hand and push the right down tube lever down a single click, then put your hand back on the bars. The bicycle will shift into a slightly higher gear.
Brake lightly when you wish to slow down or stop. When slowing down with down tube shifters, take your right hand off the handlebar and brake softly with the left. Pull up on the right shifter one click at a time until your reach a gear suitable for the slower speed at which you're traveling. Put your right hand back on the handlebars and continue riding.