Across the Imperial Highway

Visit every named location across all 8 main regions of the game. Locked areas that are specific to side quests don't count and aren't necessary for this.

The easiest way to keep track is on your in-game map; explore everywhere to remove the grey/fog and once its clear (or nearly clear) you'll be done.

As you're working through all the main, side, companion and faction quests you'll end up getting this automatically - every location gets touched at least once in some way for at least one of those quests. I.e. no special effort required here.