Assuming you have the software installed, the best place to start is with one of the Crib Sheets.
The installation contains crib sheets in the idl/themis/examples folder. These give end-to-end examples of how to load, process and plot the data available for a given mission and instrument. Cribs for general purpose plotting using tplot may be found in idl/ssl_general/examples/.
The following provide detailed information on the SPEDAS software: