Splatoon/Flooder Junkyard

Use the same path that you used to reach Inkrail Skyscape. As you reach Area 3, head right, and there are two steep ramps. Use the ramps and jump on the platform, then go right. Ink the wall and use it to reach the higher platform. Also, an Inkrail can be found near a small wall. Climb up the wall to reveal the hidden kettle leading to Flooder Junkyard.

Beginning area/Checkpoint 1
Beginning area: After taking the launchpad, you will face off against two Flooders, which cannot be destroyed. The key to the vault can be found on the left side of the area which the Flooders are patrolling.

Checkpoint 1: You will be placed in another area being guarded by two Flooders. Get to the other side and an Inkrail will be found. If you take the Inkrail to the end, there will be a crate holding lots of Power Eggs.

Checkpoints 2 and 3
Checkpoint 2: Break the normal crates to the right to reveal an iron bar. Going through the bars will continue the mission. A locked vault can be found and the key can be found past the next checkpoint.

Checkpoint 3: Ignoring the vault, heading into an area full of crates, you have to look for the key. The key is located on the right side of a back corner. You will also have to brush off a few Octodivers while trying to get the key. When the key is collected, head back to the checkpoint and unlock the vault. The vault reveals a platform with a propeller lift that will elevate you to an area with two Octotroopers guarding a Launchpad. You are not required to splat the Octotroopers to reveal the launchpad.

Final checkpoint
You will now be placed in a larger area in which five Flooders are guarding. There is a locked vault on the other side. A key is held by an Octodiver, so take it down and the key will be revealed. Be cautious though, as Flooders will still chase you. Getting to the vault, it will reveal an Inkrail. Take the Inkrail and it will lead you to the Zapfish.