  We present the steps taken to produce a reliable and complete input galaxy
catalogue for the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy
Survey (BGS) using the photometric Legacy Survey DR8 DECam. We analyse some of
the main issues faced in the selection of targets for the DESI BGS, such as
star-galaxy separation, contamination by fragmented stars and bright galaxies.
Our pipeline utilizes a new way to select BGS galaxies using Gaia photometry
and we implement geometrical and photometric masks that reduce the number of
spurious objects. The resulting catalogue is cross-matched with the Galaxy And
Mass Assembly (GAMA) survey to assess the completeness of the galaxy catalogue
and the performance of the target selection. We also validate the clustering of
the sources in our BGS catalogue by comparing with mock catalogues and the
Sloan Digital Sky Survey (SDSS) data. Finally, the robustness of the BGS
selection criteria is assessed by quantifying the dependence of the target
galaxy density on imaging and other properties. The largest systematic
correlation we find is a 7 per cent suppression of the target density in
regions of high stellar density.
