*EDIT* Thanks to MathGuy42 for pointing this out. The game may be looking for an accuracy score of 50/50 rather than actually landing perfectly centered. However, the scoring seems to be inconsistent at best so I'm not sure how helpful this advice is at the moment. I'll update this guide again once I've ascertained exactly how to satisfy the game's requirements for a score of 50 points. *END EDIT*
This was a bit tricky, but with some patience and a little luck it's quite doable. If you use this guide, I'd recommend reading the whole thing before attempting the achievement since it goes by faster than you'd think and it would be difficult to go back and forth between game and guide.
Two things to note: First, when I got this achievement, I landed with two of my wheels outside of the circle. Most of the rover was in, however. It would seem that you simply need more than 50% of the rover in the inner circle. Second, if you find yourself lower on fuel than you'd like to be, or too far from the target for comfort, it's best to just crash your rover. You'll start again immediately and avoid having to repeat the first two sections over and over.
As soon as the Landing sequence starts, raise the appropriate arm straight up to send Curiosity toward the center of the target. Don't sweat the alarms--you'll right yourself and slow down soon.
Just before or at the moment the rover is going to be dropped, raise your other arm as well. You'll level out and drift farther toward the center. After a moment, your speed will be in the green (i.e. very slow). If you do nothing you'll drift too far, so to adjust simply drop the appropriate arm completely, then immediately raise it again. This will provide a tiny boost in one direction with minimal rover swinging. Do it 1-3 times until you are fairly centered.
Next you'll want to drop a bit to conserve fuel. I found the best way to do this without going into free fall is to flap your arms up and down in unison, smoothly, not too fast, like a demented, achievement hunting bird. If you're a bit low on fuel by now, you can probably to afford to drop a bit, level out again, then start flapping.
Once you're close enough to the ground to really see where you're going to land (about when you can see the top of the inner circle), shoot both arms straight up immediately. You should be hovering just over the Martian ground. If you need to, make a *tiny* adjustment (seriously, TINY. Otherwise Curiosity will start swinging all over the place), then level out again and hover. Once you're over the miniscule center circle, flap your arms and land.
Good luck! Let me know in the comments if this helped and what I can improve.