Hi, CAT (catalase) enzyme activities have been very well studied using Native-PAGE activity staining. I have used the same for cyanobacterial CAT activities. However, it is possible to use Western Blot method as well, and I would say follow the standard Western Blot protocol while you may buy anti-catalase antibody from abcam or other source. Your standard western Blot methods will include, Preparation of protein or enzyme samples and resolve by SDS-PAGE or Native Page, then transfer the PVDF membrane by wet or semi-dry transfer apparatus, block the membrane, incubate with primary and secondary antibodies, finally develop the membrane or document the chemiluminescence and then analyse your band as CAT activity.