@rocksinmypath already gave a really good summary. But the tourism victory is imo the most complicated in civ 6 and there is LOTS more to talk about and consider. A while ago I wanted to post a small guide for cultural play in Civ 6 in the german civforum - it ended up with around 7500 words and 14 pages.
My general tips (additional to the ones already mentioned):
- focus on one or two methods (tile improvements, great works, national parks, wonders, relics and so on) to generate tourism and try to max out any bonus (GPs, wonders, Cards etc) you can get for those rather than doing everything
- build up a faith economy to buy GPs, naturalists and/or rockbands, it also helps to speed up your early game with monumentality, you dont need to get your own religion for this to be impactful
- NEVER ever kill a civ in a cultural game, the ammount of tourism you need to generate a tourist is based on the number of major civs that started the game, if you kill a civ this wont change the ammount you need but you have one civ left where you can get our tourists from, if you need to go to war at least let them live with a few small cities, if a major civ dies it might even be a good idea to go to war to revive them, in general wars should be avoided though
- if there is just one civ with lots of domestic tourists you can use rock bands in their territory to not only generate lots of tourism yourself but also lower their number of domestic tourists
- pick a civ that suits cultural play well
- practice cultural play alot, the faster you can bump your tourism up to significant numbers, the less time the AI will have to get their cultur up