White flag tourism exploits noncombatant status of land units. As such, it is usually only possible after a war with the enemy. Against pagans, you might initiate the war specifically for this purpose; otherwise it's usually too costly in badboy and/or stability, so you'll only want to do it as a coda to a war declared for other reasons.
Here's how to do it: when you are about ready to make peace with an enemy which has one or more provinces you want to explore, you intentionally move some of your own armies into enemy terrain. (Of course, you may already have units there sieging or fighting as a part of the war.) Then you make peace. Any armies in enemy terrain are then placed into noncombatant status, which allows them to move freely within the enemy country they are in. Instead of moving these armies straight home, you use them to explore the enemy country. They can stay there indefinitely.
It is very occasionally possible to get white flag tourists in via some other means than your own war. If you have military access with a country, sometimes you can predict the outcome of a war between it and a third country well enough to place tourists in the third country.
Note that this tactic will be especially cheap if implemented using tiny armies. Even a huge country like China can be fully explored by a handful of tiny armies in the 5 years a truce lasts, and at practically neglible cost.