If you have a simple key, of which there are three in the base game, you can just walk right in. Smashing it open reveals a maze-like room that leads to the Junk Pit. You're going to head toward the Flukemarm, the large Fluke-birthing boss in the Royal Waterway, and look for a crack in the room just outside her lair in the ceiling.
The main area of the Godmaster DLC is a zone called the Junk Pit. Luckily, we played through some of it to lend a hand. Much like the games it takes inspiration from, Hollow Knight hides access to the DLC in a way that isn't necessarily easy to find. Hollow Knight got its last piece of free DLC this week as an update to the well-received indie game.