Emerald Graves is a quite big location and toy can start exploring it right after you appear in Skyhold for the first time. An important piece of information is that it is fully optional to explore the Emerald Graves. This means that traveling here is not required to push the storyline forward. Personally, I recommend against skipping this location, because you can complete side quests here, reach many interesting places and obtain quite valuable items.
The way to unlock the location: On the war map, you need to select the "Meeting at the Emerald Graves" operation and complete it. It is possible only if you have, at least, 8 Power points.
Recommended experience level: It is best to travel to the Emerald Graves not earlier than before you reach experience level 13-15, because you will have to walk around many of the regions with stronger opponents. Just like usually, get prepared for the battle against the great dragon at the Emerald Graves and try not to attack him, if your party members have not yet progressed to level 19-20.
1) While exploring the Emerald Graves, you get to visit two major locations that you explore on separate maps. The ones in question are Chateau d'Onterre in the Eastern part of the map and Villa Maurel to the South. There are side quests connected with those places and you can find descriptions for them in the walkthrough for the game (as well as detailed information on how to get to those places). Another important location in the Emerald Graves is the guard's outpost to the West of the map. Seizing this location, not only gains you a new foothold, but it also unlocks the option to barter with Gertrude, among others.
2) Explore the Northern part of the Emerald Graves as the last one, because this is where the dragon's den is located. Fortunately, around, you do not complete any side quests so, you lose nothing if you postpone reaching the den.