https://www.youtube.com/watch?v=_F73KHImFSE&ab_channel=JDFlightSim

Above is a video walking you through it, but here are the basics:
1. Press AP button in upper right to turn on auto-pilot.
2. Press ALT to lock in current altitude.
3. Press HDG button to turn on heading following, and use knob to the right of the pilot seat to adjust degrees.
4. To descend or ascend to a given altitude, use the altitude selector between the two seats and then go to the center autopilot controls. Select VS for vertical speed, and use the scroll wheel next to the VS button to select how fast you would like to descend/ascend.
5. Press the AP button again to disengage autopilot or standby to get it to disengage temporarily.
6. YOU still control the throttle even with autopilot on. Be cautious you do not go too slow or too fast. I suggest keeping the throttle at half while the autopilot is on, and lower it even more if descending quickly.