When you start a new game, it will ask you which difficulty you want, choose Custom.
I'm currently doing this on Master difficulty with custom settings. There are some custom settings that actually make this a bit easier I think. Your custom settings have to total 700 points or more for this achievement to unlock at the end.
My custom settings are as follows:
Chapter Saves Only (Manual saves disabled) - 180 points : OFF
No Focus (Focus ability disabled) - 50 points : ON
Stealth Takedowns Only (You cannot perform combat takedowns) - 50 points : ON
No Reticle (Reticle disabled for aiming) - 30 points : ON
Speciality Arrows Only (Only Blunt, Water, Fire and Rope Arrows enabled) - 50 points : ON
No Food or Poppies (Food and Poppies are not consumable) - 100 points: ON
Zero Damage (If you take any damage, you fail the mission) - 60 points : ON
Expensive Resources (Cost of resources are increased) - 70 points : ON
Slowed Movement (Overall movement is slowed) - 45 points : OFF
Iron Man (If you die or fail a mission, you lose your entire game and have to restart from the beginning) -250 points: OFF
No Upgrades (Non-critical tools, gear upgrades, and trinkets are disabled) - 100 points : OFF
No Kills or Knockouts (If human/animal is harmed in any way, mission fails) -170 points : ON
No Alerts (If human/animal detects you, mission fails) - 145 points : ON
Total: 725 points
With these custom settings activated, you obviously have no focus to use, no reticle, etc. But I feel like these settings actually kind of make this playthrough a bit easier. There is no wondering if you've been spotted or have alerted anyone, because if you have, the mission automatically fails and you restart at a checkpoint.
This is going to come in handy, especially going for:
You will also in the course of things, end up with the following achievements, because you have no focus and cannot knockout anyone without failing the mission:
The A Moral Victory achievement in THIEF worth 516 pointsFinish the game without a single kill or knockout The Clear Headed achievement in THIEF worth 157 pointsReach the final chapter without using the focus ability
It has been noted to me that you can only reach the 700 or more points if you play on Master difficulty to begin with, so you will also get: (Credit/Confirmed By: PuckJunkie)
The Hard Times achievement in THIEF worth 283 pointsFinish the game on Master difficulty
You should also note that, failing a mission because you are detected or break one of those custom settings, will only fail and give you the option to start from your last checkpoint. It will have no overall effect on your Ghost status for that mission. I actually failed a few times on the second chapter, restarted from the checkpoint and finished the chapter, to find that my Ghost status was still there and I had not been detected.
If there is any information that I'm missing, let me know.