First of all, I want to give a quick definition of belly landing. While the one wheel landing is pretty straight forward as the name suggests, the belly landing consists in an emergency manoeuver performed when both your bomber wheels are destroyed and you need to land on the plane belly. It's a dangerous manoeuver both in the game and in real life. During WW2 some belly landing were performed and who did them was considered a hero due to the fact that they managed to save all their crew members and sometimes even the bomber itself.
Enough history for today, let's get started on how to perform both manoeuvers in game.

When only one of your wheels is out of commission, the landing is pretty hard to manage but still completely performable saving all the crew. The steps to follow are these:

• As soon as you reach the friendly territory (the place where you can speed up time), order your tailgunner and mid gunner to bail out. Give them a parachute and let them jump off the plane. If you are playing USA AF (as shown in the video footage) order all 4 back gunners to bail out (if you have 4 parachutes; if you don't prioritise the tailgunner and all the ones you can at the back. Remember to remove the crew member from the ventral or ball turret).
  
• Transfer all the fuel to one tank using the mechanic pump. Transfer it to the tank positioned on the wing with the wheel, leaving the "no wheel side" with an empty tank (for example if your left wheel is missing, transfer the fuel on the right tank). This step is to balance the bomber weight and leave minimum weight on the side with no wheel.
  
• Make sure that all the remaining crew members are full health or near full health. If not, heal them before landing because they will be heavily damaged anyway.
  
• If you couldn't bail out all the gunners, make sure once again that the ventral turret is empty. If the upper turret is free, put one gunner up there.
  
• Before landing, activate the lean fuel mixture, it slows down the bomber and helps softening the ground hit.
  
• Now for the hardest part: the landing itself. Lower your gear to be able to land; as soon as the only wheel touches the ground, raise your gear back. This must be performed with a great timing, otherwise, if you raise too early the plane will land too hard, if you raise too late it will have the time to tilt on one side.

If performed correctly your crew will be completely safe. The worst case scenario is that some of them will go bleed out and have the timer, but you are already at the airbase, so they will be safe. The ones who bailed out will be safe anyway because you are in a friendly territory.

https://youtu.be/6vjCUTUoQp0 (for some reason the preview is not working, I leave the link)

This is the worst landing you can possibly perform (except for a crash landing somewhere behind the enemy lines). Follow these steps to minimize losses:

• Order your back crew members to bail out (again, prioritise tailgunner). If you are playing USA AF, order all the 4 back gunners to bail out if you can. Make sure you removed gunner from the ventral turret. If the upper turret is free, put one crew member there.
  
• This step is required also in the one wheel landing, but is not as important as here: MAKE SURE YOU DROPPED ALL THE BOMBS STILL IN THE BOMB BAY, otherwise they might explode when your bomber's belly touches the ground.
  
• Make sure that all the remaining crew members are completely healed, they get damaged pretty hard when touching the ground.
  
• Make sure you lower your gear (even if it's not there, you need it lowered to enable landing at the airbase).
  
• Before landing, activate lean fuel mixture, it will effectively slow you down. Don't move fuel between tank, you need stability and all the fuel in one tank will add weight to the side.
  
• From this moment on, everything is based on luck. There's no way to control the landing, pray that your pilot doesn't do anything stupid.
  
• In the worst case you will have to change 3 or 4 crew members on a total of 7 (or 9 in USA AF), but the gunners will be safe, so they don't need re-training.

Bomber crew 101: useful tips & tricks to survive your WW2 bomber simulator

Here you can find guide updates when they come out.