The achievement unlocks when you activate the 175th lorestone out of 195 available. The achievement description says you must discover 'all' lorestones. This is not accurate. You only need to discover 175. The disparity can be attributed to the fact that there were 175 lorestones in the older XBox 360 version of the base game: Kingdoms of Amalur: Reckoning
. DLCs added another 20 but the unlock requirement for the achievement remained at 175.
Lorestones appear on the mini- and local maps when your detect hidden skill is level 10. Since this is the most time consuming achievement in the game, it is worthwhile to max out your detect hidden skill as soon as possible. Since you cannot advance to level 8 by investing a level-up skill point until your overall level is 16, a good approach is to use a fateweaver. Play the game until you reach level 16 and invest in the skills as you see fit.
At level 16, go see a fateweaver and reset your points. First, bring your detect hidden skill to level 10 and apply the rest of your points as you see fit. When you continue playing the game, all lorestones will be the most prominent feature on your mini- and local maps identified by bright blue dots. You can then activate them as you come across them without going much out of your way.
The walkthrough shows all lorestones on the map. The ones on the world map are shown by region and sub-region with special maps for the lorestones in dungeons. Even if you are not using the walkthrough, you might want to consult the special lorestones page to assist you: Kingdoms of Amalur: Re-Reckoning Walkthrough
The very short video below shows the 174th and 175th stones being activated. Note the bright blue dots on the mini-map. The achievement unlocks, after a short lag, when the 175th stone is activated: