-And so it begins...
"Make your first kill."

-Oh, you press F1!
"Start the second level."

-Half-mast hard-on
"Complete level 10."

-Get a girlfriend
"Complete the game."

-Z for Zachariah
"Complete the game having killed everything."

Be sure to triple check every level, just to make sure that no one is alive.
-I swear, the AR is for hunting!
"Complete the game using only the M16."

Not hard, just tedious. Never use anything other than the default weapon.
-Boondock Saint
"Complete the game without letting any non-hostile NPCs die"

If an NPC kills a non-hostile NPC it counts, at the end of the level subtract the hostiles count from the population from the start of the level. If the population count at the end of the level is equal to the result, you know you killed every hostile NPC without letting the others die.

There are reports that you can use the cheat code theresnoplacelikeoz to skip to the air force base level and get this achievement by completing it without killing the ostrich

These are only obtainable during certain levels.

-The Hell's Fargo Wagon
"Firebomb the band."

Level 4: Parade of Disasters. Throw a molotov and burn the marching band (you can also use other fire weapons)
-NOPE! Chuck Testa.
"Fight an ostrich."

Level 10: The Farm & Level 16: Air Force Base. Kill an ostrich.
-Never nude-ist
"Kill a naked person."

Level 4 (Special Delivery expansion): La Palamino Resort. Kill one of the naked people.

The achievements you will probably get by the time you finish the game:

-Forrest Gump
"Run for 5 minutes."

-Patrick Bateman
"Make 100 kills."

-Dexter
"Make 1000 kills."

The achievements you will have to grind for:
-Mickey and Mallory
"Make 10000 kills."

-Holes is holes
"Take 10,000 hits."

-Peter North would be proud
"Hit 100,000 targets."

-Bulletstorm
"Fire 1,000,000 bullets."

These are going to take a while, there are AHK scripts available online that automate the process.

-Took a banger in the mouth
"Take a rocket to the face."

Get hit by a rocket, simple enough.
-I'm afraid I just blue myself!
"Commit suicide."

Press K.
-Bad touch!
"Touch someone while burning."

When you're on fire, touch an NPC.
-How Kevorkian!
"Perform your first execution."

Sometimes after you shoot someone enough they'll crawl on the floor and beg for you to put them out of their misery. Walk up to one and press X (or whatever you have bound to execution)

-Dirty knees
"Duck under a rocket."

Crouching under a rocket dodges it.

-From my cold, dead hands
"Complete a level with less than 10 health"

Pretty self explanatory
-You want our autograph too?
"Watch all the credits."

Beat the game and don't skip the credits.
-Ballmer Baller
"Play the Mac or Linux version of the game."

I'm guessing you're currently on Windows, I'd recommend setting up a Linux virtual machine. There are many tutorials on youtube, here's a random one.
https://www.youtube.com/watch?v=x5MhydijWmc
After that, install steam, log into your account, download POSTAL and play the game.
-Sissy
"Enable cheats."

Here's a guide someone else made with a list of cheats, type these in while playing the game.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1236709563

Alternatively, if you own a Steam Deck, just play the game on it, as SteamOS is based on Linux.

-Army of One
"Use every weapon in the game."

Pretty self-explanatory.

Thanks for reading my guide! If you have any feedback let me know.