This tutorial will describe the shortest & easiest method to delineate watershed in QGIS.
You just need a DEM (Digital Elevation Model) of desired area and a reference base map like Google map of Google Earth Images

Tutorial to download SRTM DEM in QGIS through SRTM Downloader plugin
   • Download SRTM DEM in QGIS  

Tutorial to add Google Images and Maps in QGIS through HCMGIS plugin
   • Google Earth Images in QGIS