How do you slow a tractor trailer using the clutch?

The only way the clutch pedal will slow you down is if you're going uphill. If you're trying to downshift, the only difference between what you describe and doing it without the clutch is the clutch itself - when shifting one of these trucks, you either double clutch or you float (don't use the clutch).

As you're braking, you wait until the tach gets down to 1000 RPMs (if you plan on dropping just one gear) or 700 RPM (if you plan on dropping two gears). You push on the clutch (only enough to engage it - you don't go all the way down with it), and take the vehicle out of gear, then you release the clutch. Once you've done this, you bump the accelerator to 1500 RPMs, push in on the clutch again, go into the gear you want to downshift to, then release the clutch again.