To unlock this achievement, you must find every Silver Medal in Sonic, Shadow, and Silver's respective ACT and HUB areas, as well as the final episode, End of the World. This is the single most time consuming achievement in the game excluding Legend of Soleanna and Gold Medalist.
The other solution listed here says that the medals make a noise when close. They do, yes, but they can be difficult to hear depending on other sounds present in the game. If you're going after this achievement without a video guide, you can rectify this by going to the in-game Options menu, turn the BGM off and the SFX up. This makes the medal chime much easier to hear.
Also, try to defeat all of the enemies in a room when you're searching, as some medals only appear when they're all killed.
If you plan to use a video guide, I recommend ShadnicK826's YouTube channel. He has video guides for every single medal in the game and goes through them in quickly and simply. A link of one of his videos is included in the bottom.
I hope this helps you. Good luck!