All the shenanigans are to be made in the Strange Place.

1) The pentagram must be activated(all the Great Old Ones must be defeated)
2) You will need to collect 3 books and have them in your inventory
3) It can be done on any difficulty
4) It can be done with any character

Go into exact center of activated pentagram.
But this is not all you have to do to place your character accordingly as the hitbox is very small and picky.

1) Locate the line near pentagram's center
2) Place your character so their feet are slightly above the line
3) Left and right borders of your character's shadow must be only mildly touching the dots this line connects; also make sure they're equally touched by your shadow
4) Face either to the right or to the left as facing up or down can mess your positioning up

This is how it looks in-game.

Right after the last step you should open your inventory and read Al Azif.

*Note: Theoretically it works by reading either of all 3, but I've always been choosing Al Azif, so the other 2 aren't tested.

If you did all the things right then you will get a teleportation SFX, same as when you use stone circle teleports.

Note: If monsters appear instead of SFX, it means something went wrong. You're not supposed to get any monsters if you did all the things right. You may notice this as the screenshot below lacks any monsters' corpses.

This is what you will see:

And then this loading screen will appear:

Eventually, I figured out how to teleport consistently, but it took a lot of practice.
Sometimes I couldn't get to the secret location despite doing all the things right.

For some weird reason, restarting the game somehow fixed those issues.
If you're sure you do all things right, but still can't get to the secret level, try restarting the game(don't reset the game's progress), but make sure you will appear in Strange Place(where the pentagram is located) when you press Continue in main menu.