  IXPE (Imaging X-ray Polarimetry Explorer) is a Small Explorer mission by NASA
and ASI, launched on December 9$^{th}$ 2021, dedicated to investigating X-ray
polarimetry allowing angular-, time- and energy-resolved observations in the
2--8 keV energy band. IXPE is in Science Observation phase since January 2022;
it comprises of three identical telescopes with grazing-incidence mirrors, each
one having in the focal plane a Gas Pixel Detector (GPD). In this paper, we
present a possible guideline to obtain an optimal background selection in
polarimetric analysis, and a rejection strategy to remove instrumental
background. This work is based on the analysis of IXPE observations, aiming to
improve as much as possible the polarimetric sensitivity. In particular, the
developed strategies have been applied ``as a case study'' to the IXPE
observation of the 4U 0142+61 magnetar.
