As the game has some long areas without nearby checkpoints or points of reference, it's a bit hard to explain in detail what point of the level a plaque is in. I'll try to make it as clear as possible, posting all the plaques in chronological order, so if it takes a while before you notice the area, don't worry, you may just not have reached it yet.

I will also reference checkpoints which you can fast travel to, but you only get the fast travel skill at the end of the first level, so keep that in mind.

If you're doing this after completing the levels, an Agile Shield to quickly blink past all the enemies helps, as there are few barriers in the game where you have to kill all enemies to progress. Careful though, as you'll have some stragglers from previous areas joining the fight as well.

NOTE: Finishing the Drivasser level is a point of no return, and will port you directly into New Game Plus. Make sure to collect all plaques before finishing it or you will be forced to start a new game to collect the remaining ones.
Fixed in patch 1.2.

From Old Market Square, progress until you see this statue:

Right behind it, there's your first plaque!



From the Royal Amphitheater, progress after the miniboss and into the catacombs. Clear out the area, and in the room right before the exit, on the left side you will see open doors that lead into a room:

Inside the room, against a sarcophagus will be the plaque.



As soon as you pick up the second plaque, you will receive the achievement Old Drivasser Explorer.

From the very beginning, Greymaw Pass, go down a slope and some stairs and to your right you'll see a small pillar:

Behind it is the first plaque.



It'll be a while until the next plaque. From The Springs, past the springs you will make your way up a slope with a building on your left side. You'll notice this vortex point:

Behind the small brick wall will be the plaque.



From Tracker's Climb, right after the final (second after checkpoint) boss fight inside the lodge, go into the next room:

Pretty much in plain view, to the left.



Grey Teeth Explorer

From Boiling Point, get past the bridges over lava until you reach an open snowy area. A little bit further, there will be this slope:

Continue and to your left will be a mammoth skeleton:

Get in close and look carefully for the plaque.



From the last plaque, continue along until you see your first ship. Climb on top of it:

Go to the back end of the ship and turn around to see the plaque.


Alternatively, you can reach the ship going backwards from the Upper Maelstrom checkpoint, for a shorter route and less enemies.


Maelstrom Explorer

From the Outskirts of Talmas, after a long and healthy walk you'll see this building. It's impossible to miss as you have to go through it to progress in the level, but it is a fair way off from the checkpoint.

Inside, straight ahead, the plaque.



From the Lordly Colonnade checkpoint, you'll have to take another long-ish walk until the next plaque. It'll be shortly after a green barrier that you'll see these stairs:

Walk down and look behind you against the wall by the stairs for the last plaque of the level.



Zasad Explorer

From the Mourning Sands, you'll be in an open desert. Following the path, keep an eye out for this formation on the righthand side of the road:

Go towards it and look to your right to see this tomb:

Enter and find the plaque among the treasure.



From the third checkpoint, Hall of the Faithful Servant, you'll end up going deeper and deeper down into a dungeon. After a couple floors, you'll end up in this room:

Continue along towards the back of the room:

You'll find this pillar with the plaque, next to the stairs you have to go down to progress through the level:



Right after the last plaque, from the Upper Grotto checkpoint, you'll leave the dungeon and be in a cavern. Just a bit further in you'll see this area, notice the the wooden cart:

By the bunch of coins at the feet of the cart is the plaque:



Lost Zasad Explorer

From the beginning of the level, Silt Harbor, continue through the town until you get to a beach, and see this structure:

To the left will be this square stone area:

Go towards and into the water, and around the corner against the wall will be the plaque.



From the Memorial checkpoint, follow the main path and you will eventually see this area:

Under the bridge, to the right will be a scaffolding:

And the plaque:



Soddentrod Explorer

From the second checkpoint, House of Sacrifice, continue until you come up to this clearing and enter the temple you see on the left:

Now ascend the temple to the top, these are the last stairs to climb:

Up the stairs, to the left against the wall is the plaque.



From the last checkpoint before the boss, Yawning Maw, after a while, you'll get to this location (in the rift ahead, you'll see the Gryphon speak of how the corruption in this place is stifling and that he will not fail you):

Go to the rift, turn around and look to your right, and on the ground will be the plaque.



Sinkhole Explorer

Progress along the obligatory route from the beginning of the level, Fallstead Burning, and you'll walk into this area:

Just ahead, on the stairs of the house to the left.



From Hespon's Crossing, after the forest area, you'll enter this cave:

Go down on the right side, and look to your left:

Against the grate will be the final plaque!



Drivasser Explorer

Achievement 9/9:

And once you pick up that last plaque, you'll get the final achievement, Archaeologist!
Congrats! [Except it's broken right now, so hope we get a fix soon!]
Achievement removed September 22nd.

A thank you to Ishtarias, Onikuma, grantmaxson, Zav, and Razafeth for helping find the plaques, as well as all the others participating in the steamcommunity discussions!

Hope the guide helps, and feel free to comment / criticize. Enjoy!