How do you down shift a semi truck?

Depends on what transmission you have, and (if you have a 13, 15, or 18 speed) whether you're trying to split a gear down or trying to shift a whole gear.

As a general rule of thumb taught in driving schools, they'll tell you to downshift at 1000 RPMs (or 700 if you're dropping two gears). When you get down to 1000, you push the shifter out of gear, rev the motor to 1500 RPMs, then push the shifter into the next gear down. If you time it right, there's no need to use a clutch pedal. Bear in mind that this applies only to unsynchronized transmissions (the most common type used in North America). European and Asian trucks use synchronized transmissions which were not designed to be "floated" without use of the clutch.