On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Hidden Journals. You can start this quest in the Sanctuary location during the completion of the main mission named Bright Lights, Flying City.
The Highlands - northern part
2) Hidden Journal 1
3) Hidden Journal 2
4) A safe position to fight with Pyre Thresher
The Highlands- southern part
5) Entrance to the Robot camp
6) Path to the upper part of the camp
7) Hidden Journal 3
8) Hidden Journal 4
Tannis  will order you to find her journals. Use Fast Travel and head to The Highlands Overlook. The first Journal is below the bridge . Go down the stairs on the left side of the bridge shown in the screenshot.
Below the bridge, head to the end of the platform. there you will find a building inside of which a Journal is. In order to get there, jump between the metal elements shown in the screenshot.
The next Journal can be found in the house marked as  on the map. As you approach the bridge, you will be attacked by a Pyre Thresher. If you retreat to the passage from the screenshot , you will be able to attack it from a safe place.
After eliminating the enemy, you should be able to find the journal on a boat, behind the house on water. Jump between the poles coming out of the water to safely reach it.
Now head to the southern part of Highlands and through Robot camp entrance gate . Head to the end and of the right you should find a path leading to the upper part of the camp . The passage to the journal will be heavily protected, so be ready for a hard fight. Once you reach the entrance to the room with the Journal , shoot the fuse box shown in the above screenshot to disable the force field.
Having collected three journals, head to the south-east part of the map, to the location marked with  on the map. The last Journal will be inside a container. Show through the grates into the faces hanging at its end to open the passage on the other side. Take the last journal and return to Tannis  in Sanctuary to collect your prize.