'...and in the structure's overgrown far reaches, many nomads wander, living off the remnants of battles fought among the many warring states and corporations...'
Metal Garden is a minimalist shooter wholly taking place within the confines of a megastructure. Since the game is heavy on immersion and exploration, it is recommended to go in blind the first time and check the guide only on a per-need basis. The logs are not tied to a single playthrough and the achievements one may miss can be gotten afterwards through chapter selection.
Half of the achievements require exploring the side paths thoroughly, thus this guide is organized into a chapter-by-chapter format, making sure not a step is skipped along the way. Note that the player character does not take any falling damage and slipping into the abyss teleports him back to nearby grounds with only a little HP loss: so feel free to explore to your heart's content.
If you are checking this guide only for the logs, search for the term Log # to cycle through them. To unlock every achievement in a single run, be sure to enable at least one modifier as you start.

KING OF NOTHING
The game utilizes a unique health system through which the player character is fully healed right after his HP drops to zero, yet he is left with an injury that results in a relevant handicap. Though lost hit points can be replenished via medkits, accumulated injuries can only be healed by using the stationary auto-surgery units found throughout the megastructure.
In an unmodified run, this achievement can be unlocked right after confronting the initial trio of enemies. Deal with them, then pick up your first grenade from the open locker where one of the mercs was standing guard.
Move on and let the next wave of mercs around the corner drop you to a quarter of your health before gunning them down. Once you reach the king's encampment, throw the grenade at your feet to get injured, then treat the said injury by utilizing the auto-surgery unit found nearby.
Log #1: From the top of the hill overlooking the encampment, move towards the leftmost leo (i.e. the berserker) laying dead on the ground and interact with the datapad beneath him.
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THE NEIGHBOURHOOD JUST GOT MORE INTERESTING
The weapon case holding the sniper rifle can be found to the back of the shoreline encampment, by the foot of the bunker overlooking the area in question. It first needs to be unlocked though.
Mind that following the shoreline path signified by the rock arch leads to the end of the chapter. Instead, take the metal door located below the highway to step into a rather lengthy side area.
The path eventually comes to end with a ladder by a subterranean waterfall. Ascend the ladder to find yourself in the bunker from before and pull the lever within, then take the adjacent door and get the sniper rifle from the weapon case, unlocking the achievement in the process.
Log #2: Before taking the shoreline path, jump onto the roof of the bunker and interact with the closed vent cover to reveal a stash of newspapers and magazines.

TERMINOLOGY OF SCALE
Log #3: Stepping into the dome area, explore beyond the roadside wall to the left to come across a singular car and interact with its trunk to discover a datapad.
Accessing the elevator room further on requires sending up the elevator through a lever found in the first side area of the chapter. In that regard, rather than the making your way up to the top of the dome, explore the opposite side of the ground floor to find a bunch of interactable leaves.
Push them aside and drop down to find yourself in a short C-shaped corridor that connects to the underground tower area. Here, you may opt to pick the charge rifle toting mercs from afar or hop down directly to get their help for the following achievement, if need be.
This achievement requires leading one of the leos (i.e. the berserkers), preferably heavily injured, to either type of mercs' line of sight so that he gets killed through friendly fire. The set piece by the tower provides ample opportunity for doing this: just keep dancing around the charging leos until one of them is taken down by the charge rifle mercs sniping you from the tower.
Log #4: Once the battle is over, look for a trio of crates near the tower to find another datapad as well as some supplies to keep you going.
Instead of ascending the tower, look for a rather large opening by the foot of the waterfall to drop down. After some platforming, you arrive at a seeming dead end where an ancient key resides by the bars of the waterway. Pick it up, then interact with the slightly indented part of the wall to the left. Go through and pull the lever directly ahead to send the elevator on the other side up.
After that, retrace your steps to the opening, then either use the key at hand to remove the bars by the waterway on the other side of it or make your way back up by utilizing the broken pillars.
Return to the dome and battle your way up. Once at the cliffside, take a moment to check the path to the right before jumping onto the giant cogwheels to cross the abyss. Get the elevator room key residing on the crates and take the awaiting elevator down to finally reach the other side of the store room from before, unlocking the achievement in the process.
Note that the elevator room key is only used for opening the door that connects this side of the room to its other half from before. On another note, the second side area of this chapter can be reached by taking the opening to the right of the cogwheels instead of the apparent path right ahead. The area in question revolves around a giant puzzle room, yet exploring it is not needed as long as the achievements go.
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A WASTELAND HIKE
Even though this chapter does not feature any achievements in itself, there are still two keys to be collected for unlocking a later one and yet another secret log to find.
Log #5 / Dam Armory Key L: Starting out, take the opening found to the back of the windmills to step into the first side area of this chapter. Once you reach the underground encampment, look for a parchment laying on a rock by the campfire, then pick up the key in question residing on a crate by the opening that takes you through the waterfall back to the windmills area.
'No heavenly fields awaited us beyond the structure... and no God to fill our empty stomachs.'
Dam Armory Key R: After clearing out the narrow passage that leads out the windmills area, look for a red-coloured vent cover to the right and interact with it to find yourself in the second side area of this chapter. Go all the way through to end up at the overlook where a charge rifle merc was standing guard before and pick up the key in question residing on the rock.

THAT WHICH ESCAPED THE NOTICE OF MAPMAKERS
The gate to the armory in question is located right ahead by the start of the penultimate chapter.
Considering you have both of the keys from the previous chapter in possession, interact with the card readers found on the sides of the gate to unlock the armory as well as the tied achievement.
Reading the sixth and the last secret log found in this chapter should unlock this achievement.
Log #6: Leaving the arena behind, look for a piece of machinery among the trees on the left and interact with the datapad residing at the foot of it.
Once you are confronted by the 'Border Watcher', instead of shooting directly at it, aim for either one of its dual rocket launchers or the core hatch found on its top until that module is destroyed.
Note that the achievement can still be unlocked even after defeating the boss by destroying any of the modules in question once he is down. -- Credit goes to Andras for the addendum.
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PLANET SURGERY
Reach the end of the game either on an 'unmodified' or a 'modified' playthrough.
These two achievements for completing the game can be unlocked at the same time, on a first run, as long as at least one modifier is enabled. -- Credit goes to 13xforever for confirmation.
For unlocking 'Megastructure Amble, Side B' after an 'unmodified' playthrough, you can opt to replay the last chapter (instead of the whole campaign) with at least one modifier enabled, thus saving some time and effort. -- Credit goes to Bluellama and KatAdAstra for the addendum.
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'Finally preparing to leave the elevator, and facing an ever stranger landscape, you nonetheless find the familiar step of your nomadic wandering returning: the promise engraved in every mile traveled that the seeking itself is the answer.'