The Video Guide is right above as it shows all the collectibles for every area in the game.
The following is a written guide for everyone's enjoyment.
The Squats: CollectiblesCollectible: Fajar - The Botanist: Not Missable
*** Spoiler - click to reveal ***After completing the prologue, you should be inside your dojo, where your character should finish fighting. Turn around and proceed to the desk in front of the window to retrieve the item. This is not to be missed.
Collectible: Skull Letter + Avenue Door (Locked): Missable
*** Spoiler - click to reveal ***You would enter the level through a red door, which would give you two options to take: one to climb up a wall, and the other to your left, which would lead to stairs. Take the route to the stairs, but once around the curve, you'll see two entrances. Instead of going up the stairs, walk through the left door and grab the Skull Letter and the door itself for both collectibles.
Collectible: Purple Mist: Missable
*** Spoiler - click to reveal ***Once you have defeated all of the adversaries at the top of the stairs. The character tells you to run across a ramp. Once doing so you will then be confronted with more adversaries within the apartment. When you're finished, look to the left side of where you jump down; in the kitchen area, right next to the refrigerator, there's a bag containing the drug Purple Mist.
Collectible: Bills: Missable
*** Spoiler - click to reveal ***After defeating the huge guy, turn and stroll over to where he was seated and get the Bills collectable right next to his seat or the piles of money that's knocked on the floor.
Collectible: Club Flyer: Missable
*** Spoiler - click to reveal ***After passing via an elevator shaft, you would enter through a door that would lead you directly into a room of guys that you must battle, or to the left where there is a room with a table and a dangling lamp above it. The Club Flyer has been put on the table for you to take. Both paths do make you battle in the room of guys.
Collectible: Metal Gate + Bunch of Keys
*** Spoiler - click to reveal ***After exiting the last collectable area, you should enter a corridor with a lock gate. Interact with the Locked Metal Gate collectable to obtain it, then turn around and enter the chamber in the center of the corridor to battle two foes and obtain the Bunch of Keys collectible.
Collectible: Purple Flower: Missable
*** Spoiler - click to reveal ***You'd walk into a room full with plants hanging from the structures and all around you. With a lot of guys to battle. Because this is the most in the entire level. So, instead of going through the blue door, which would lead to another fight, turn around and look for the Purple Flower on the right side of the table.
Collectible: Keycard 206 + Private Room Door (Locked): Missable
*** Spoiler - click to reveal ***You'd reach the last door after dashing down the alleyway. When you enter the room, the Keycard 206 is right in front of you on the table. After picked up, proceed to your right, not up the ramp to the level's boss, but to the right of that, behind the tarp, lies the Private Room Door, which is now closed and must be unlocked later in the game once we have obtained the Flower Key collectable.
Return Collectibles: Collectible: Hongbao Flyer: Missable
*** Spoiler - click to reveal ***Need: Flower Key: Sanctuary: Missable
Following the acquisition of the Flower key from the Sanctuary Level (Last Level). You can go to the door at the end of the level that was previously locked. Upon inside, a red Hongbao Flyer is waiting for you on the table in front of you.