How do you down shift a tandem truck?

It's going to depend on your transmission. If it has a manual transmission, it's most likely unsyncronised. When your RPMs drop to around 700 - 800, you'd take it out of gear. Since the transmission isn't synchronised, you'd then have to step on the accelerator to tach the motor up to around 1500 RPMs... if you're fairly new to this, you might instead tach it to 1700 - 1800 or so, then let off the accelerator, so as to 'catch' it at 1500 as the motor revs down. Once the motor's at 1500, you'd go into the next gear down. You can even drop multiple gears... you might wait until the motor drops to 500 RPMs, then do the same thing described above to drop two gears at a time... if you ever drive local (local delivery, dump truck, etc.), you'll learn this technique for sure, as you'll probably more shifting in that line of work in one day than you would in three or four days of over-the-road line haul driving.