How do you perform a gear-up landing in a large airliner?
Assuming that the pilots have tried everything possible and the gear just does not like to come down:

The general rule is to find a long runway, touch down as slow as possible. Here are sections extracted from Boeing 777's Flight Crew Training Manual:

Dumping fuel


  If time and conditions permit, reduce weight as much as possible by burning off or jettisoning fuel to attain the slowest possible touchdown speed.


Elevator


  After touchdown lower the nose gently before losing elevator effectiveness.


Rudder


  At touchdown speed, the rudder has sufficient authority to provide directional control in most configurations.


Speedbrakes


  (...) speedbrakes should be extended only when stopping distance is critical. Extending the speedbrakes (...) may compromise controllability of the airplane.


Reverse Thrust


  (...) an engine making ground contact could suffer sufficient damage such that the reverse mechanism may not operate. Selecting reverse thrust (...) may produce an additional asymmetric condition that makes directional control more difficult. Reverse thrust should be used only when stopping distance is critical.


Landing technique

The technique varies with the combination of failure. If one side of the main gears is missing, land the plane to the side of runway which corresponds to the extended main gear. I.e. if the left gear is down and right gear is up, land on the left side of the runway instead of centerline. Maintain wings level for as long as possible, and be prepared to brake on the left side to maintain directional control.

If all the gears decide to hide in the gears bay, land on centerline and use rudder to maintain directional control. Rudder is effective until airspeed drops too low.

After landing

Evacuate the aircraft as necessary!