I used a vacuum laser level to get the top height right, then a small hand held level to make sure the vertical lines were plumb.
It helped using one of these low voltage drywall mounts - they have tiny holes in the corners to help cut out the exact dimensions required for mounting.
That means a lot to me - I'm learning as I go here and trying my best. :)
Also, this is the level: Ryobi AirGrip (Home Depot). It's probably not great for a daily driver level, and I'm not sure of the longevity - but it's great for a hands free, no damage level.
Oh wow, that's neat. I could see that being used on new construction where the drywall screws aren't caked behind 7 layers of paint. I would be spoiled by that tool.