You can't miss the alter. It looks like a skeleton hand sticking up through the ground, about the size of the largest buildings you encounter in large villages.

The locator stones spawn inside large furby villages and their weird alters. Look for 'man made' rock formations or clear every village you can find. Or just scour the plains for the hand.

If you are OK with such things, the map generator online can also tell you where it is, or where the hint stones are, or both... If you want to do it the hard way, well, its hard... this one can take a while to get lucky