How to enable multi-camera in Premiere *after* I've already made tons of cuts I started with an hour-long video interview and cut it down into a 4-minute sequence of clips using Adobe Premiere Pro CC 2017.

Then I remembered that I had a 2nd camera's video file.

How can I convert my sequence to a "multicam" sequence even though I've already cut it up a bunch?

I want to sync up that 2nd video file so that all of the cuts are the same and that I'm then able to simply bounce between 2 camera angles throughout my 4 minutes of cuts.
Ryan, sorry, but I think it's basically too late. You can certainly turn on multicam editing mode now but Premiere is not going to recognize your existing edits. If you have more footage to edit you could use multicam mode for the remaining portion.