For SONARNL (ie just with sonar) you'll need to start up Mapper3Basic
and then select 'New Map' and then create a map yourself with a tape
measure.  Mapper3Basic should show you how long a given line segment
is as you are making the segment, segments can be made to snap with
the snap tool or by pressing control.

You can change maps used by a server (such as guiServerSonar) at run time 
by connecting to the server with MobileEyes, and entering the new name as
the value of the Map parameter in the Files section of the Robot 
Configuration dialog.

Note that if you have a robot with a laser a map created with the most
recent Mapper3, it will contain the line information needed to use SONARNL
and in a much easier fashion than making a map by hand.  Just follow
the directions in Mapping.txt if this is the case.