This way of leveling isn't for everyone, without the right materials your group won't be able to make huge XP.
First of all this is in act1, quest 6 (The Broken Blade), part 2 (Talk to Alaric) - You'll need to get the checkpoint from the crypt of the ancients in festering woods.
I would highly recommend you to run this with a group of 4 players, preferable with a monk, but doable without. What you aim to do is clean the crypt fully and restart the game on as high torment as possible for the DPS you have in the group. You should aim to take 20-30sec per run on avg.
The crypt has a fairly high chance of spawning XP pools, always has 1 elite, very often a shrine and possible 2 small chests (This run isn't for you if you aim to farm legendarys!)
We peak xp/h on torment 4, but if you can blitz the crypt in the same time with t5/6 do so.