You don't. You stop the aircraft in a position of your choice. And a little box should appear, assuming you've completed all mission objectives saying "Mission complete: OK/Cancel". Clicking OK should take you to a groovy cut scene where you see the pilot doing something like getting out the plane, dancing the hula etc and then it should go to the debriefing screen.

Turn off either the fuel or magneto's. The magneto's are either CTRL+M or SHIFT+M or a combination of the two. Check the key functions for the fuel.

Woodlouse, genius that you are, I have to disagree with your description of when the "mission completed" box appears.

What normally happens, in my experience of flying Heavy Bombers, is that if I've successfully bombed my target and flown the waypoints until I'm on a pretty good final towards the runway; just at that crucial moment, when you're concentrating real hard on speed, angle of attack and altitude, a ruddy great red stamp is smacked into your vision, telling you that the mission is successful, and would you like to continue, or to debrief.
Whilst you're reading this, you're still heading for the tarmac, so it's one hell of a distraction!

If you choose "debrief", you're taken to the debrief screen and don't complete the landing. If you choose "continue", which removes the red stamp,  and successfully land, then basically you can shut the plane down, or just hit the escape key and select "END MISSION" from the pulldown panel which appears at top right.

After landing, I usually raise flaps and taxy off the runway. Then I lower the throttles to zero, set the park brake,  pull the props to minimum and set the mixture to lean. At this point the engines stop. You can then set the magnetos and battery off as Woodlouse says, but apparently on some aircraft you were supposed to leave the engine running and open the bomb-bays for bombing up the next mission. Hand cranking the bay open took forever.

After the engines have stopped, I usually pop the canopy, and enjoy the silence.