Quote Originally Posted by oscroft View Post
... Then if, for example, you've chosen f8, turn the focus so the infinity mark lines up with the right-most 8 mark on the d-of-f scale, then look at the left-most 8 mark - this indicates the closest distance that is sharp at f8. ....
That's an instruction in Hyperfocal focusing.

The other way is to set the aperture you want, then make a wild guess at the distance to the subject and set the focus to that.

If you want a sharp horizon, focus at infinity. If you think something in the foreground is more important, focus on that. Do not try to get both in sharp focus by using DoF markings - you will only get both disappointingly not-quite-sharp.