Intensity normalization using average global mean for pet images

Hi. I need serious help. If someone helps me I would be thankful to him/her.
I have PET raw data and I am pre-processing using spm12. I first set the center of each subject data and then used the MNI template spatial normalization of the data. Now, I want to apply intensity normalization using the average global mean method but I cannot understand how I can apply this. Please someone can guide me.