How does one start a listener trace?

Use:

lsnrctl trace 16  (to turn the trace on) 
lsnrctl trace off (to turn the trace off)

Or set the following listener.ora parameters and reload:

TRACE_LEVEL_LISTENER=16
TRACE_FILE_LISTENER=listener.trc (listener.trc is the default)
TRACE_DIRECTORY_LISTENER=$ORACLE_HOME/network/trace

NOTE: 'LISTENER' is actually the name of the listener in the above example. It may be different on your system. View/ edit your listener.ora file to get the names of the defined listeners