The Karma achievements are made significantly easier with the addition of the Broken Steel perks.
The first step in getting all three achievements is to create three save files at level 28. The closer you are to reaching 29, the better, but at any point will work fine.
Getting to level 30, not the karma, is actually the hardest part of these achievements. If you are like me, and reached level 20 long before the game ended and before you got Broken Steel, you have a long way to go after the added quest ends. The first things I would do is the side-quests that are offered surrounding Project Purity. Return to the Jefferson Memorial and offer to help Scribe Bigsley, who is inside the building. That will result in two decent quests. There is also a side-quest that can be accessed by hacking or a successful speech on Bigsley or by talking to the ghoul Griffon in the Museum of History.
Once those quests have been competed, you will probably still have a long way to go. The fastest way to make up that experience is by hunting Super Mutants. After finishing the Broken Steel quest-line, speak to Paladin Tristran and ask if there is anything else you can do to help. He will offer you a reward for killing super mutants and bringing back blood samples to him. This is much easier than scouring ruined buildings for scrap metal, pre-war books, cameras or sensor modules. The National Mall and the Germantown Police station are great places to hunt mutants. Just make sure to get the well-rested bonus before you trade in the samples. 10% extra experience goes a long way in this grind.
You can also increase the difficulty to get more experience per encounter. This is really a must if you are playing on easy or normal. The extra 15 or so EXP an encounter will really speed things up in the long run. Also remember to leave your companions at home. They are experience hogs. You won't receive any EXP from enemies that they kill.
Once you get there, load up one of those files and, when you reach level 29, choose Here and Now. This perk will immediately level you up again, to 30. At level 30 choose Devil's Highway. This will set your karma to Very Bad and you will get the achievement.
Rinse and repeat for the other karma achievements. I recommend that you don't use this method for whatever karma type you mainly play as, as it really is a waste of 2 perks.