Mapping/Tutorials/Levelflags

General

Click View->Filter->Entities to hide all entities
Hide all stepon and clipping brushes (select them and hit h) (hint: Shift h will unhide all hidden brushes and entities)

It should look like this - the stepon and playerclip brushes should not be visible anymore

Now select the first level brushes by holding shift + left mouse button and drag

After you selected the 1. level you should hit the s-key to open the surface inspector. Select all levelflags from 1-8 like seen below

The same for the next level - make sure, that you also select the ceiling of the 1. level but not the ceiling for the 3. level

Select all but the 1. floor levelflags to show theses brushes in all but the 1. level

Just to complete the steps do the same for the 3. level.

Mark all but the first two levelflags

The highest level selectable when playing on your map is set in the worldspawn. it is called maxlevel. see Mapping_For_Dummies/Lesson1#Set the map lighting in the worldspawn for details of setting worldspawn flags.