PLEASE NOTE - this post is "pinned" as a Top Question. If you have something relevant to add, then please go ahead and post on this thread. However, if you have questions, I'd politely ask that you start a new thread, so as to keep this one "clean" and uncluttered.
OK, so, here goes:
For the FMM tourist visa, this cannot be completed until you are within 30 days of your arrival in Mexico. The official, Mexican government website is this one - https://www.inm.gob.mx/fmme/publico/en/solicitud.html
Upon arrival in Cancun, the Customs/Immigration officer will stamp this document and tear part of it off and return it to you. (You might want to score through the paper or otherwise make it easy for them to tear it.) Do NOT lose the part they return to you; you will need to surrender this little chit of paper when it's time to leave Mexico.
If you do lose it, here's information on what you need to do - https://www.tripadvisor.ca/ShowTopic-g150807-i8-k11618131-Replacing_a_lost_immigration_form_tourist_card-Cancun_Yucatan_Peninsula.html
FWIW, I know of at least one website that offers to fill this form out for you .... for a fee. It is absolutely NOT necessary to pay anyone to do this for you; it's quick and easy to do yourself. I'm also aware of another website, "cancunairport" that has a form there for you to complete. However, as that is NOT the official, government website, I wouldn't use them myself. ("cancunairport" is, I believe, a transfer company with a slick, and arguably, rather deceptive, website.)
The online Customs Form can be found here - https://siat.sat.gob.mx/PTSC/DPE/ Several months ago, I saw quite a few reports of the printed online not being accepted at the airport but don't recall seeing any such reports recently. I believe that one terminal at CUN no longer requires Customs Forms (presumably they have kiosks but I'm not sure about that). In any event, the online Customs Form cannot be completed until three days or less prior to your trip.
If you prefer to fill out the forms handed to you on the plane or available when you land at the airport, that's still a viable option. I'd suggest that you make sure you have a pen and your passports handy if you want to go this route.
Happy travels to all!