Excellent advice from skyboi. I'd like to add some comments and a caution or two.
Do set vehicle on jack-stands if you need the clearance to work and really need to wash undercarriage well. No problem with that. However, use only a garden hose if possible. if not, at least be aware of the following...
SAFETY: Pressure washers have very long wands as a safety feature, not just convenience. Use of a nozzle, directly in the gun is actually violation of industry standard practices. Sometimes, it is the only practical method though. The length helps prevent the gun from turning around on you and causing a water puncture wound. They can be serious, or even deadly. This type of wound was virtually unknown until when diesel fuel injection was invented, and now more common with pressure washers. Ours at work is 3,500 psi at 4 GPM. Consider even the 2,000 psi home owner versions can rip off your fingers without mercy. You are trying something in close quarters, more so than just washing off the outside at a comfortable distance, so just use a bit of extra caution. Protect your eyes with goggles. (skyboi is right on!)
CORROSION: Secondly, when you do spray underneath, be careful of any electrical connections that would normally not be exposed to pressure washing, putting water into them and causing corrosion later on. Steam Cleaning is notorious for this. You are fairly safe worth just pressure, but look around first, that's all.