If the aircraft is pitched down greater than 20 degrees pull power to idle, level the wings (before doing anything else), and pull up.

If the aircraft is pitched up greater than 20 degrees, push the throttles full forward and push the yoke forward to bring the aircraft level to the horizon. (both can be done at the same time)

Keep the aircraft level, don’t do anything with the autopilot, it’s a lot more complicated than just a “button” and if you don’t know what you’re doing it’ll hurt a lot more than it helps. If turning the autopilot off is necessary, it can be turned off by pressing the button labeled “AP”, or simply overpowering it with the control stick/yoke (this will turn it off automatically). Though it may be better to just leave it on if it is already on.

The pilot has or had a headset on, put it on yourself, place the mic as close to your mouth as you can get it. The here tells you to switch to the emergency frequency 121.5, but all commercial airliners are flying IFR, so generally speaking whatever is already put in will do if you can’t change the frequency. The tail number of the aircraft may be placarded inside of the cockpit. If you know the tail number, start with that, give as much information as you can as quickly as possible. ATC will give you directions, and/or put you in touch with someone who is an expert or instructor for the specific type of aircraft you are in.

Things to keep in mind:

Pitch for airspeed, power for altitude. If you want to climb, increase power. If you want to descent, decrease power. If you’re going too fast, bring the nose up, if you’re going too slow, pitch down.

Jet engines take a while to respond to changes in power, this is especially important when landing.

If a landing is not going well, increase power to full, climb, and advise ATC that you are “going around”.

Going into specifics about landing is pointless, procedures are complicated and different for different aircraft and different situations.

To the people who say that “you’ll crash and burn”. There have been studies done by sticking completely untrained people into a simulator and giving them instructions, and it is possible for an untrained person to land an airliner, but at the same time, there’s no way a guide on the internet is going to be able to effectively teach you how to do it.

That said the scenario where you would need to is unlikely, as there’s a good chance there’s a relief crew (on larger aircraft/longer flights), an airline pilot that is deadheading, or at least a private pilot that’s traveling with you.

(AOPA has an excellent article on this called “Pinch Hitting a King Air” but as far as I know the story is in the password protected department)