The SExtractor home page http://terapix.iap.fr/rubrique.php?id_rubrique=91/ contains detailed documentation including the SExtractor User's guide and SExtractor for Dummies.
The following examples illustrate running sextractor
1. run sextractor on a given image with a configuration file
sextractor sky.fits -c config.sex2. run sextractor to create a default parameter file
sextractor -d