[Freesurfer] Warping to MNI space

Hello,

I plan on using the aseg.nii to create a grey matter mask. I want to warp these grey matter masks to MNI space. I have been trying to use @auto_tlrc in AFNI to do this warping but am having some trouble.

The reason I need this in MNI space is because I plan to make a group grey matter mask that I use to restrict my funtional fmri data analysis, which is in MNI space (MNI_avg_152T1).

How might I go about doing the warping and I wanted to double check the order of doing things? Should I create the grey matter masks first then warp to MNI? Or do I warp the aseg files to MNI and then create the grey matter masks?

Thanks so much!

Emily