Welcome -> Start the game.
Work again -> In the singleplayer mission get caught by the man of the house and try again.
WOW! -> Pick up a can of gas and throw it (hold LMB) so it explodes.
Surprise -> Go to the kitchen and eat a pie or dogfood, this will fill up your 'present'-bar. WWhen it's full you can press 'Q' to poop and the achievement will pop.

Friend -> start a multiplayer game

As a dog:
Ahhhhh -> throw an item at the human so he falls.
FLASHBANG -> spit a human in the face (aiming is a bit hard) so he temporarely can't see.
Not yours -> play on the map vacation, enter the house and climb on the bank so you can try to get the gun on the small table next to the bank. You can't actually use it as a dog, but when you press 'E' while looking at it the achievement will pop.
Objective complete -> Win by killimg the human.


As a human:
Officer down -> as a human, throw an item at the dog so he gets pushed awway.
Pleasant flight -> as a human, kick a dog at high power so he flies through the air.
I AM HUMAN -> Win by killing the dog.

General
-By throwing items through windows you can create very usefull escape routes for when you get chased.
-There is actually no way to kill the human in singleplayer.
-If you're outside and the human sees you you can just run around the house until he gives up following you. (although it maybe a bit faster to just get caught and respawn)

Missions
find the key -> Go upstairs, go to the next room in the lower left drawer of the brown cupboard
take a pet's place -> downstairs through a door you haven't went through before and click on the dog on the chair
suspect room -> Upstairs, next room, next room
something weird -> a note on a cabin in the room you're in
book -> upstaris in the middle room, a blue book on a cupboard
after book -> go in the garden, look for a mudpool
code -> In the kitchen, on a note on the fridge.
what key is for -> Outside, apply the key to the lock of the shed
rusty lock -> In the garage, get the wrench from the table, apply the wrench to the lock from close up by climbing up the shed

During the story mission I got a bit desperate looking for the key so I thought, let's make a guide to help others out (if anyones still playing this).because it didn't seem too much work. I hope it helped you.

PS: I'm now done with this game so if you're from the future, feel free to ask me questions still, but there is a big chance I won't even remeber this game.