These steps explain how to create an LOD group if you already have the various versions of a mesh you want to display at different times. If you only have the high resolution version of the mesh and you want to automatically generate the lower resolution versions to display, see Generate LOD meshes.
For example, view the scene through one main view panel, and select your main game camera from the Panels > Perspective or Panels > Orthographic menus.
Select the highest resolution model first, and the lowest resolution model last. By default, objects are assigned to the LOD level stack in the order you select them, so you want the highest resolution model to be ordered first.
For example, selecting (1) Sven_Body_HiRes, (2) Sven_Body_MedRes, then (3) Sven_Body_LowRes creates the following LOD group:
By default, the Threshold Type is set to Screen Height Percentage so that levels display based on the percentage of screen height taken up by the overall LOD group in the current camera frame. Screen Height Percentage measures the overall height of all the meshes in the LOD group relative to the current camera frame, and establishes an initial default percentage for each level to display. You can edit these threshold values yourself later on. See Edit LOD threshold values.
This creates the LOD group in your scene, and hides all but one level in the group, depending on your camera view.
Expand each LOD_n sub-group to see the associated mesh (or meshes).