The standard way of calling eis_prep is:
IDL> eis_prep, filename, /default, /save
where filename is the name of the level-0 FITS file. The /save keyword tells eis_prep to create the level-1 FITS file and associated error FITS file in the working directory. Additional keywords that you may want to use are:
/quiet - eis_prep runs without popping up information widgets.
/retain - retain pixels with zero or negative DN values. See Step 2 in What does eis_prep do?.
/noabs - calibration is not performed, so the final units are DN.
/photons - instead of erg/cm2/s/sr/Angstrom, the final units will be photons/cm2/s/sr/Angstrom
The full list of keywords is given in the header of the eis_prep routine.