I highly recommend spending time with the guides on this site and info in the forums. This one (K1 Fiance Visa Step-by-Step Guide on Filing an I-129F for a Foreign Fiance) covers this stuff pretty well. I don't believe anything at this stage (the filing of the I-129F) needs to be notarized, I didn't notarize anything on our I-129F.
Just to clarify, for the I-129F they ask for Evidence of In-Person Meeting, not "proof of relationship." Later in the process (getting the greencard) you'll do a separate, different "proof of relationship" exercise.
The in-person meeting bit is on the I-129F form - Part 2, Question 53, "Have you and your fiancé(e) met in person during the two years immediately before filing this petition?" and then question 54 gives you 5 lines to describe it.
I used the guide here on VisaJourney (here's that link again) to prep everything. On line 54 I wrote a short description but then put SEE ATTACHED, and created a separate document that was included in the I-129F I submitted. I just followed the guide, and made a cover page with a brief description, and then wrote out details of our two most recent in person visits, and on the subsequent pages, put photos and captions from those in person visits. This was a word document that totaled 5 pages, and then I also attached as much documentation as I could as part of this packet - boarding passes, receipts from tours we went on, hotel bookings (thanks to reading up on this site, I made a point of putting BOTH our names on the hotel reservations so they would appear on the receipts/folios).
Hope that helps! Best of luck, it truly is a journey... a long one for sure, take a lot of patience and research but this website is a tremendous resource.