There's a lot of ways to do this, first I'd say raise a softcore and a hard core character to 70. The softcore character should do this by at least beating the game (all 5 acts) so you unlock adventure mode and torment difficulty. The hard core character can do this anyway way, they count for Dream team, so getting that out of the way if you're going to do it will help this.
When you're ready to level your characters, there's two ways to do this. The fast "acceptable" way. Or the "jerk" way.
The Acceptable Way:
Set your game up to be torment level 6. adventure mode with your leveling character. Check Act 1 to see if Jar of Bones, or The Matriarch's Bones are available (at the cemetary). In act 2 You want to do A Miner's Gold. In Act 5 You want to do The Miser's will. If any of these show up you want to run them.
For the Act 1 bounties, you will not get the bounty, but you want to do the event.
While running them you will die.. A LOT, this is expected as it's torment 6. however the game will give you 16x your normal reward for running the difficulty. This is gives you about a level for each bounty/event. 5+ levels for the initial run.
So die and try to force your way through the bounty/event. as you level up you'll gain some abilities that will let you fear, teleport or jump around the level, use these wisely (potentially using elective mode) to try to stay alive and avoid dying as much as possible.
Note: A miner's gold will complete with out you, if you are dead when it completes, you do NOT get the reward, so timing when you're alive is crucial there... Or just staying alive as long as possible.
After you complete the bounties or if none show up, you'll rinse and repeat. quit the game and continue again.
The "Jerk" way.
NOTE: I DO NOT ADVISE THIS AS THIS IS A BIT OF A DICK MOVE! but... it's possible.
There is another way. Rifting at torment 6 creates a TON of XP, at the 16x bonus. If the game level (who created the game) was set at 70, you will get even more value. So what you need to do is take you level 70 character, quick match with a game, switch to your low level after quick matching (but before officially entering the game) and then enter the game. If someone is in the rift, (their banner will light up you can click on their banner to be warped to them) and they are fighting, you're in a good game. If no one is in the rift, it's likely the rift is already complete, or no one can survive it. Join the person in the rift if you can and run around behind him, sucking up the XP. You'll level... FAST.. even at level 60-69 you'll see the xp bar go up as fast as if you were level 1. The key to this however is again to stay alive (no xp if dead) and be alive at the end of the rift, (when the rift is turned in you'll get at least 1 level, and more likely 3-4 levels) . You'll level faster than you ever imagined.
However since you are making the game harder, not pulling your weight, you will be screwing over the people fighting in the rift. They may only do one rift (if they start another they might be powerful enough, hacked enough or something else, feel free to stay) or quit in the middle. That's why this is a "jerk" move. I don't advise this but it will likely stay available long after hotfixes come and go.
Note: Blizzard is in the process of hotfixing the game so the bounties in the first method will not show up, so the jerk way may become the only way to do this quickly.