Sonic Frontiers

For each of the four islands, there are what the game calls Challenges that you need to complete in order to reveal your map. These are the little white diamond signposts you'll come across as you explore, and they are shown on your map/compass as a red square icon. Each island typically has 25-30 of them to complete, with each revealing a few squares of your map. Thankfully, you'll encounter a handful naturally as you explore and do each story objective, and completing a couple is typically enough to reveal more on your map. Worst case, if you don't see one on your map, head to the area of your map that's missing and keep an eye on your compass on top of the screen. The icon will show up there when you get somewhat close, making these pretty easy to track down and complete.

As for the challenges themselves, they vary quite a bit, but in general are quite easy. They range from drawing a circle of light () around objects, to climbing/scaling pillars, to doing little puzzles of button pressing, to short races. I don't think a guide for these is needed, as none will push the limits of your abilities. I completed them all first try without any prior knowledge or guide.

The only annoying thing about these is that some require it to be nighttime, and there isn't a way to make that happen other than waiting for night to come. So, as you explore and work on open world achievements, make a mental note of which red icons on your map are nighttime ones (I believe they may have a litte 'N' icon on them to indicate they're nighttime challenges, but make a note to be safe). Then, when night falls, make a beeline for those nighttime ones to knock them out.

Thankfully, this achievement tracks, but you might as well track it with your map itself. Once you reveal the whole map for each island, you'll unlock the corresponding achievement.