First you must make sure that all the objects you want are colored gray. The default lambert material has the best results. Your model should be sitting on a floor of some type. This floor should also be assigned to the default lambert.
Step one - Create an area light from the rendering shelf.
Step two - Select, move and scale and rotate the area light so it's shining down and it's larger than the object.
Step three - Open the attributes editor (window>attributes editor).
Step four - Roll down until you find the tag shadows and then the ray trace shadow attributes tag.
Step five - Check use ray trace shadows. Set the shadow rays to 40 (you can get by with less) and set the ray depth limit to 9.
We're going to be using Mental Ray as our rendering software. First we have to set some camera attributes.
Step one - On the view port menu go to view>select camera.
Step two - In the attributes editor roll down and find the tab environment. Set the background color to a light gray. Mental Ray will use this background color for sampling Final Gather.
Step three - Now we have to open up the render settings. Click on the screen icon.
Step four - Set the render using to Mental Ray. Now click the tab Mental Ray in the render settings.
Step five - Set the quality preset to production. In the section Secondary Effects, check Final Gathering.
Step six - Now we need to set some Mental Ray attributes for our area light. Select the area light and, in the attributes editor, roll down and find the tag Mental Ray, and then area light. Check use light shape.
Step seven - Set the high samples to 200, and the high sample limit and low samples to 100.
Now render your scene. If your rendering takes too long, try lowering the Mental Ray attributes of the area light and lower the shadow rays and ray depth.