Once you have everything done, you can do the iris scan as a drop-in (no appointment needed) at the locations that have it. My interview was at the Detroit (Ambassador Bridge) center, and I just set aside some time during my next trip to YYZ to get the scans done. (Entered "the old way" using my passport, asked the officer where the NEXUS office was, and went on over after I'd finished clearing through. Easy.)