I'm going to list them the same order as Steam shows them on achievement page.

(I'm going to use here the same format used by *Elisav the Evil, I hope they don't mind I borrow it ;) )

The guide follows the following format:

Description: The Achievement's official description.
Type:

• Normal Progression - You will get it during any normal play through of the game.
  
• Missable - You might miss it unless you explore and/or do something special
  
• Exclusive - Getting this achievement prevents you from getting a certain another achievement in this play through

How to get it: Instructions on how to get it.


All credits must go to whom is due. Any help received will be added with due credits.

I had meant to put credits for help in each achievement, but going to thank Wingyl here for helping with several achievements. :)

Description: More like... Finished the "game".
Type: Normal Progression


How to get it: Simply "finish/beat" the game. :P Joking aside, this achievement is given when you finish the Intro map.

Side note: If you play the demo first and then continue your progression in the full game from where demo ended, you will get this achievement so soon you start in the full game.

Description: What kind of boss fight was that?
Type: Normal progression.


How to get it: Icon says all. "Defeat" the Sky-b-a-s-t-a-r-d. Poor Coda.

Description: Your Words Go Here
Type: Normal Progression.


How to get it: This one it's not hard to get, simply ship your map. In other words, really beat the game. xD

Side note: A fun fact is, despite the description may point out that (at least for me) it might be related with Story entities in the Level Editor, actually it is not. According with devs it's like nobody was able to really come up with a name for an achievement given as reward for finishing the game, then it remained as a Placeholder :P

Originally posted by KainAsylum:

Placeholder Achievement #99 is like saying 'TODO: Write something meaningful here as reward for finishing the game'. The joke here is that we, the real developers, never quite finish tha-"

Credit: Thanks Sir Prize for helping and KainAsylum for explaining it.

Note: Both achievements below are mutually exclusive. Getting one prevents you from getting the other on the same save. Make sure to backup your save to get both achievements, devs have provided info about the save files here. Also both are normal progression, you can't progress without getting at least one.

Description: Remembered your first when it mattered
Type: Normal Progression, however Exclusive


How to get it: Do you remember where you first met Pro? Yes, the Menu. During the "Overworld nightmare", simply go to Menu (Return to Title). Also, you can get the achievement by exiting the game like through Exit to Desktop. The game will restart at the proper place when you boot it back up again. (Thanks Kain for pointing that, I could take that hint :P)

Description: Old Pro had to do it for you
Type: Normal Progression, however Exclusive


How to get it: Do the opposite you should for Back to where we met. Just faff around while Pro freaks out until he opens the Menu for you.

Note: Both achievements below are mutually exclusive. Getting one prevents you from getting the other on the same save. Make sure to backup your save to get both achievements, devs have provided info about the save files here. Also both are normal progression, you can't progress without getting at least one.

Description: Got enough of Starfather's speech to make a proper mixtape
Type: Normal Progression, however Exclusive


How to get it: Simply sit through all of the Ish's speech in E4. (Poor Ish...)

Description: Pushed the button because you got bored
Type: Normal Progression, however Exclusive


How to get it: Do the opposite you should for Starfather. Kill the feed (skip) before Ish has finished that speech. "I dare you, push the button!" :P

Description: Embrace the null state
Type: Missable


How to get it: As the achievement icon hints, just walk a certain distance as "ghosted". Officially, you must walk 1000 meters while "ghosted" but since there is no way to measure that, usually walking around for about 5 minutes should be enough.

EDIT: As pointed out by sammyrigdon496, actually you can check the distance walked by looking at the achievement description in the Achievements page.

Description: The boundaries of this simulation have no hold on you.
Type: Missable


How to get it: The achievement icon says it all, but description can mislead. See that big hand-like structure at the center of the map? Yep, just jump off from its top. The description says about "boundaries" but if you jump on the void you may not get this achievement. Just jump off and land on anything.

That structure is visible in the intro map but it's unreachable. You can try the achievement in overworld, after you have acquired some ally with flight ability (preferentially Juggodillos or Rocks, that are ones that you can hop onto). Simply guide your flying pet to the top. It can be tricky as you may not be able to waypoint depending on the angle.

By the way, there is also a little secret at the top (related with another achievement) ;).

Description: Hell hath no fury like a tester scorned.
Type: Missable


How to get it: This one is interesting and contains some lore. Simply get all the hidden change logs by Lief Troper. There is a nice guide about where to find them here. You also will get a nice "gift" at the end :P.

Side note: Quality Assurance can help in getting Master Scout. The things you need to get Quality Assurance are pointed out by giant purple arrows that are only visible while ghosted. Run around as a ghost, and you'll find them!

Description: You will be known as the person who made that thing.
Type: Missable.


How to get it: Simply thing: get a 10/10 rating from Pro in the Level Editor ;) Ok, the objective is simple to describe, achieving it is not. Luckily, the devs were kind enough to post what is needed to achieve it here (spoiler warning).

Credit: Thanks Sir Prize for pointing it.

Description: This can only be...
Type: Exclusive


How to get it: The achievement icon seems to hint it. Just to quote dev's words:

There is some irony in the fact that the way you get that achievement is by starting a new game and resetting your save file.

Then yes, to get this achievement you can risk to lose some progression. However, luckily the devs has shown where the save files are and then maybe you can make a backup of it if you don't want to lose some progress. Info about where the save file lives at can be found in this sticky thread.

A note though, it's obtained when done after actually finishing the game. Thanks decimus28 for pointing it out.

Description: There is no keypad here, but you did it anyway
Type: Missable


How to get it: You have to input 4 numbers during game play. They originated in a popular first person game. They are a reference to a sequence of numbers used to unlock a vault in that game and and in turn is a reference to the novel Fahrenheit 451. The numbers are 0451.

Credit: Justin and some page on Web :P.

Description: From this point on, you will have to exhaust your own content.
Type: Missable


How to get it: Simply put: Find everything. EVERYTHING. Every Change Log (except those marked as Additional, those aren't accounted for this achievement), every Audio Log, every Developer Commentary, undelete each deleted content... Also edit all the creatures, although just getting into their Mindspace (edit mode) is enough, no need to change their behaviors.

Side note: if this achievement doesn't pop up so soon you get your last item, simply open Inventory and the achievement should pop up. Or simply keep playing, it will eventually show up.

Grabbing all items related to this achievement may help you later in the game. The more items you get, more life you will have available in the Level Editor.

EDIT: While helping and with the help of Trash Panda I have come to realize this was somewhat incomplete. Therefore, follows some lists about how many or where to find the elements needed for this achievement:

Undeleted Objects
• 1 Pro Head by the Origin Tree, touching it behind it.
• 1 Pro Head near the Origin Tree, down below if you jump to the right near where you start.
• 1 Deleted Cave Entrance on start island.
• 1 Pro Head in the aforementioned Cave.
• 1 Rock Path near the first crack.
• 1 Ghost Bridge.
• 1 Pro Head in the area above Starfather statue.
• 1 Pro Head hidden at the wall at left near the entrance to the forest.
• 1 Pro Head at the area above and at right of the entrance to the forest.
• 1 Pro Head in the crumbling tower.
• 1 Pro Head in the upper forest area, near the entrance.
• 1 Pro Head in the upper forest area, near the village gate.
• 1 Village cave entrance.
• 1 Pro head outside village fence, past the village cave entrance.
• 3 Space dungeon sections.

There are 3 Pro Heads that introduce Audio Logs, but you can only fill one (the other two are auto awarded), so only one will count up. They are:
• 1 in the cave above the first Howler if you go left at the Starfather statue.
• 2 in the forest, one on either side of the big tower.

Edited Creatures

These are the amount of editable creatures in Overworld (no specific order):
• 9 Flamers
• 17 Hivers
• 1 Hiver Queen
• 5 Howlers
• 4 Juggodilos
• 1 Key
• 8 Mushrooms
• 1 Mushroom Wizard
• 1 DoorBot
• 8 Rocks
• 1 Securitron
• 1 Cerebra
• 1 Sentinel
• 1 Sky-B-a-s-t-a-r-d (Thanks Steam for this spelling :P)
• 10 Corpses
• 3 Cyber-Rats
• 3 Teleport Pads
• 1 Tractor Bot
• 3 Whirly Birds
• 1 Hatchworm
• 1 Hellhound (This one is only available after completing another achievement)

For the other items, there is a good guide about where or how to find them here.