Cyclotron Radiation Emission Spectroscopy (CRES) determines the energy of beta particles from their cyclotron radiation frequency in a magnetic field. Since the cyclotron frequency depends on both the particle energy and magnetic field, variations in the field can affect the reconstructed beta spectrum. Understanding and controlling these variations is therefore important for the He6-CRES experiment and its search for physics beyond the Standard Model. In this talk, I will discuss our work to characterize and improve the magnetic-field homogeneity within the He6-CRES decay region. I will introduce our approach to magnetic field mapping and active shimming, and discuss the development of the shimming system used for He6-CRES.