Inside you'll find an enchanted apparatus next to Sandal. In Hightown, go to the marker that denotes Hawk's Estate. Be sure to read its section on the DLC page to make sure you do everything correctly. If you have the DLC, go to the chantry and starts the second quest, it'll end with you having him in your party. If you gave Anders Heroic Aura and Haste I'd take him, along with Fenris for the other two party members. A few companions (like Anders) need a certain amount of friendship/rivalry to do things, so being friendly with everyone would be nice, but not strictly necessary.
Note if you have the Exiled Prince DLC you need to max friendship/rivalry with Sebastian, so I will point out those choices. As I mentioned early on, romancing Isabela ensures she sticks around for quests we need later, so for this walkthrough I'm keeping her in the party, and the only dialog options I will tell you to choose are ones that avoid her getting rivalry/allow romancing her (unless choices are needed for quests and stuff). Your party from now on is your own to choose, with a couple caveats. So, now we have the 4 friends achievement.
