These achievements unlock naturally as you make progress through the game. If you complete the main story, you should obtain all of them automatically.

In Pursuit of Loath Nolder
After the first nightmare, Arthur will call you. At the end of the conversation, you get the achievement.

Field Agent
In the painting in Nolder's office, you will find a letter from Clark Field. In it, he shares his home address. Solving the underlining puzzle unlocks the achievement.

Descent into Darkness
Begin to descend into the well in Clark Field's house.

Open and Shut Case
This achievement triggers by unlocking Clark Field's briefcase. SPOILERS: The code is 1998.

Calendar Boy
This achievement triggers by entering the date of Clark Filed's secret meeting at Ivar's house. SPOILERS: The date is 07/05/11.

Like Clockwork
Solve the gear puzzle in the third nightmare. SOILERS: A screenshot included in a section below contains the solution.

Sleeping Sickness
Unlocks automatically at the end of the third nightmare. Howard looks at the clock and discovers that he had slept for two days.

Friend or Foe?
Unlocks automatically after the conversation with Loath Nolder at the deepest point below the trap door.

Instrumental
Unlocks after completing the fourth nightmare. You will need to play an instrument. SPOILERS: The necessary order is 2 3 1 2 1 2 4 3 1 2 1 2 1 4. Note that the first button is closest to you and the fourth is at the top of the screen.

Man of Letters
Underline the code "kd78g78" in the letter in Ivar's cabin. Then SPOILERS: dial 5 3 7 8 4 8 8 on your phone.

Map It Out
Unlocked by successfully marking all of the local locations from the notes in Loath Nolder's desk onto the map in the basement of Ivar's cabin.

Not Standing Still
Unlocks after successfully opening the second wooden statue found behind the locked door in Clark Field's house. SPOILERS: The solution is in the screenshot in a section below.

Lord of the Rings
Achieved by unlocking the box with the three rotating rings in the fifth nightmare sequence. SPOILERS: To solve, keep rotating the rings until all three pegs line up at their respective circle tops.

Househunter
Received after taking the phone call from Arthur about the house associated with the decoded phone number.

Open Sesame
Unlocked by solving the tunnel door puzzle in the temple under the Braunbell mansion's small building.

Tomb Finder
Unlocked by revealing the cemetery on your travel map.

Line Them Up
Unlocks after solving the tomb door puzzle. Solution is in an image in a section below.

Family Business
Find the Loreid medallion during your second visit to the underground tomb.

The following achievements require additional work or searching beyond that needed to finish the game.

Something's Not Right
At the start of the game, after ending the call from Arthur, check the answering machine. Call Arthur back to ask about the missing messages.

The Onlooker Sees All
Look at the calendar in Nolder's office.

Dread Me
On the second floor of the Field house, inspect the documents in the inbox. Underline the secret clue "Christopher T. Dreaden" to unlock the achievement.

Shattered Psyche
During the second nightmare, go into the bathroom and look in the medicine cabinet. You'll find a red insect. The achievement will then unlock. This achievement may also require several inspections of the two-headed graffiti during the first nightmare, as the description suggests.

Sometimes Time Flies Backwards
During the second nightmare, after turning off the radio for the second time, look at the clock.

In Memory of HP Lovecraft
Enter 05/31/37 or 08/20/90 (date of birth and death of Lovecraft) into Clark Field's organizer. You'll see a photo of H.P. Lovecraft.

Persuader
When leaving Ivar's cabin for the first time, you'll be stopped by a strange man. He will ask whether you read his note, you should answer "You left that note?" (Requires the "Dread Me" achievement.)

Nevermore
At the Braunbell mansion, left of the entrance to the small building, is a crow. If you click on it five times, Howard will quote from a poem by Edgar Allan Poe.

Trust No-One
This achievement requires two steps. First, in a document in your office, underline "do not go to the police." Second, in the leftmost letter in Ivar's desk, underline "police friends." (If you find you are not able to attempt this, you most likely need to change your game's difficulty settings.)

Backstabber?
This achievement requires quite a bit of prior "secret solving" by combining thoughts and ideas in the inventory. It unlocks once you manage to combine "Police Friends" "Old Fool" and the dialogue with Arthur about the Braunbell phone number. If you need help getting to that point, please refer to my separate Secrets guide (the process is a bit involved). (If you find you are not able to underline in certain documents, you most likely need to change your game's difficulty settings.)

Hidden Message
After being attacked below the small building on the Braunbell property, return to your bedroom and check your answering machine messages.

Poor Braunbell
This achievement also requires quite a bit of "secret solving" by combining thoughts and ideas in the inventory. Specifically, I think you must have managed to deduce "Ivar Burgen Killed Clark Field" before you can begin. If you need help getting to that point, please refer to my separate Secrets guide (the process is a bit involved). You must then speak to the prisoner below Ivar's cabin if you have not done so already. Then, after having woken up from the dreamcatcher nightmare, return to the letters in Ivar's desk. Underline "E.B." This should cause you to deduce that the person in question is Edward Braunbell. Then, underline “little building in the garden is under our control.” This should unlock the achievement. Speaking to the prisoner again after this results in a new sequence.

Please do not read any further unless you want to see the solutions to some of the game's main puzzles. The solutions are shown in chronological order without explanation below. Credit for these screenshots goes to vilfor who first published a Russian version of this guide.