How do you use the jake brake on a truck with an automatic transmission?

Your transmission should have a "manual" or "hold" mode; use that to stay in the gear you want while using your jakes on downgrades.

When you get your truck back, open the hood and look at the data plate on the side of the engine. It'll tell you what the engine's governed speed is, keep your RPMs below that number. It should also tell you the RPMs for peak torque and peak HP, which you can use to help you drive more efficiently.

Note that the engine's governed speed might be different than the red mark on your tachometer. My engine's governed speed is 1700 RPM but the tachometer's red mark is way up at 2500 RPM, which is way too high for a modern (downsped) 15 liter diesel.