Treasure Maps are a bit like a scavenger hunt. They bring hunters together, are fun, and can be very rewarding.
In order to get a Treasure Map, you need to catch a Relic Hunter. It is just nuts about ancient mousy artifacts from the days of yore, so in order to catch it, you‘ll have to find yourself an Ancient Relic first. Certain mice drop these, they are all pretty tough to catch and they don‘t drop a Relic every time. A list can be found in the MH Wiki.
You can gather as many Ancient Relics as you like (they show up in your Collectibles page), but each time you catch a Relic Hunter, one Ancient Relic will be used and is subsequently gone. The Relic Hunter is always on the move, it only stays in one location for 24 hours at a time. The MH Timer in the Useful Links Chapter can tell you where it is at any given time. Catch a Relic Hunter and it‘ll drop a Treasure Map Scroll Case. You can only have 20 regular Scroll Cases, and you can only work on one Treasure Map at a time.
A different approach is to get the region specific Scrolls. These can be bought at the Cartographers in the different region in exchange for Ancient Relics.
Open your Scroll Case and you‘ll get a Treasure Map that needs you to catch certain mice who hold clues to the whereabouts of the treasure. You don‘t have to do it on your own, you can invite up to 4 friends to help you with your task.
Each mouse has to be caught only once, and only by one team mate. Not everybody has to catch every mouse.
Incidentally, you can skip everything you read earlier and just get invited by a friend, because that works as well. Upon completion of the Treasure Map, every team member gets a Treasure Chest which contains gold, points and a lot of helpful goodies. You even get a Chest if you didn‘t catch a single mouse from the list. You can also join, leave the team and join again, provided there are still mice left to catch and provided you’re not the only person on the map. In that case the map would be completely discarded. If you are the map owner and you leave the map, ownership is randomly transferred to somebody else on the map. There is no way to remove people from the map unless they leave voluntarily.
In the regular Scrolls there are different kinds of Treasure Maps which give different kinds of rewards, and there‘s no knowing which one you‘ll get until you open your scroll. The rewards are, first of all, tied to your rank. Lower ranks get less than higher ranks, obviously.
The different maps have different levels of difficulty. Easy and medium ones can be mastered by all the ranks, although you‘ll probably have to catch some SB+ only mice. The tougher maps will need the participation of at least one high level player, as they call for the toughest mice of many of the regions. These maps will often take several days to complete.
Furthermore, maps can either be common (and all are at first) or rare. To transform a common map into a rare map, you have to sprinkle it with Rare Map Dust, which can sometimes be found in your Scroll Cases. It can also be acquired through donating or bought on the Marketplace. A rare map has the same difficulty level as the common map it started out as, the mice on the list won‘t change. The loot from the Treasure Chest you‘ll get upon completion will be better and more numerous though, so it‘s a nice bonus.
If you feel you‘re unable to complete a certain map, you can always skip it for good. Just start from the top and get a new one that hopefully is more to your liking.