Glitch Daddy is another acid trip of a first-person adventure from Sad Mask Party that is more akin to their debut Mango than the follow-up The Lotus Holidays due to its linear flow. As with the developer's previous games, it keeps a mysterious and immersive aura of its own, thus it is recommended to delve into it blindly and explore at your own pace, without referring to the details provided in the further sections of this guide.
That being said, it is possible to get all the achievements in a single playthrough, as long as an extra step is taken early on (see 'Trash Completionist' in Secrets). For the least time-consuming yet the most spoiler-free run, first have a look at the details of that achievement then keep the ordered list below at hand for reference. In case any of them are missed, it is easy to unlock the other achievements tied to secrets as the game autosaves at the start of every level and keeps them all in the load menu.
Here is what to look for level by level, listed in order of progression (as well as basic tips):
Note that the dialogue choices do not affect the outcomes, so go through them as you wish.
'Sometimes I feel like I'm on the edge of finally figuring out what it all means.. And then I'll get distracted by the silliest thing, and just like that, all the progress I've made is instantly gone.'

These should pop up at key points during a first run. The points being;
once the title screen comes up after talking to Glitch Daddy (in Intro),
right after your Bestie executes his plan (in Tropical Island),
once you solve the machinery puzzle that opens up the way (in Silent Theater),
as soon as you overcome the first boss (in Caves),
once you punch the correct code into the machinery (in Chapel),
and as soon as you manage to deal with the second boss (in Facility).

Most of these are tied to thorough exploration and interaction. Details are as follows:
Intro: Once you drop down the hole in the basement and offered a choice of two paths, take the red-lit one to the right first to meet an unusual fellow and talk to him.
Harbor: Before getting in the sub that takes you to Tropical Island, follow the beach umbrellas all the way to the end and interact with the bottle laying on the shore near the last one.
City #2: As per description, simply get hit by a car. This one restarts the level, so do it early on.
City #2: As you pass by the playground, you are asked by French Guy to collect the trashbags around the city and offered $5 in return for each. Even though two are enough for progression, there are a total of four to be found. Getting them all is not only required for this achievement, but also for 'Full Meal' further down the line, which in turn paves the way for the secret ending, 'Death to Glitch Daddy'. Refer to the marked map above or the shots below if need be.
Check the coded paragraph below the shots, for the extra step needed for the secret ending.
1/4 - Leaning on the outer wall near the benches at the back of Memphis Tower.
2/4 - At the far corner of the tennis court found next to the Silent Theater.
3/4 - In the garden of the villa found to the south of the Travel Agency.
4/4 - Leaning on the back wall of the yellow coloured villa on the way to the cemetery.
Future City: As you do the first time during Intro, opt to the red-lit one first as the path is divided into two in the basement and say hi to the unusual fellow.
Caves: After powering up the machinery, return to the main cave and check the alcove (marked above) between the two gates found on the path that connects the West and North sections.
Chapel: Find Confessions Room at the very end of Tropical Corridor, get in and confess to Mouth.
Facility: Dive into Infinity from the door on the first floor and keep walking towards the horizon.
Facility: Once you reach the Strawberry Maze, explore it thoroughly until you come across yet another strange fellow and talk to him.
Facility: This achievement prerequires 'Trash Completionist', as well as the associated extra step.
Down in the Strawberry Maze, find the room with a bed and a table laying on the corner. Interact with the plate on the table to place the strawberry you carry along next to the other two. Explore the maze further to find the vending machines just around the corner from the alcove of floating strawberries. Interact with the one in front to get a cola with the $5 you have on you from way before. Finally, return to the room in question and place the cola on the table.
Do not forget to get the white fence keys that appear on the table before leaving the scene.

Recommendation: get the main ending first, then load your last save and go for the secret one.
Shortly after leaving the Strawberry Maze behind, you find yourself in the Orbit. Dropping down the hole in the final room and simply following the path laid out before you results in the main ending, 'It Worked!?'. The other option is to turn around as soon as you land in the white corridor below and unlock the fence with the relevant keys (see 'Trash Completionist' and 'Full Meal'). Following this alternate path grants you the secret ending, 'Death to Glitch Daddy'.[www.imdb.com]
Hopefully this guide adds to the enjoyment. Feel free to comment if further assistance is needed.