Nuclear Waste
This one is fairly simple: You get it when you knock down all the cardboard figures in Homicide. There are 3 of them on the first floor, 2 on the second. It is recommended to clear the level first before knocking them down.

The Bar of Broken Heroes
This one you get by playing the intro of Subway. Go to Writer's phone and interact with it. Complete the level as you normally would, but here's the catch: Instead of leaving, he goes to a bar to talk to Biker(Who apparently survived the first game). You can see basically every main character of HM2 at the bar.

Stare Into The Abyss
Ever wondered why you don't get the achievement for not killing anyone on Writer's levels? This one is why. Play Withdrawl and clear the third floor. You will get picked up by the 50 Blessings guy and get taken to an office. When he goes to get "coffee", left or right click and you'll see Jake pick up something. Afterwards, when he gets killed and you get switched to Writer, walk up to the corpse and interact(Again, left or right click) with it. You'll get the thumb drive Jake picked up and go to a hidden level.

Well, that's it! Hope you enjoyed and hope I helped! Please post feedback on if it's confusing, if it's helpful, what I can change, etc.