Step 1. Have 3 high level characters. Step 2. Run nightfall and both raids each week on all 3 characters. This equates to 15 exotic chances every week. Step 3. If you still have time during the week to do things, you can do PoE, Trials, Crucible, or Dragon Strikes. These are less preferred options, because PoE requires keys, Trials requires you to do well, and the other 2 require quite a bit of luck. Do this enough and it's pretty easy to get all of the exotics