After installing the device management tool, go ahead and plug in your phone and make sure USB debugging is on. LG's software should detect your phone and offer to install a USB driver if the connection can't be made. The update should pop right up and give you the option of downloading and flashing.
Just like a regular OTA, you need to be stock for this update to work. It shouldn't delete anything, but back up important stuff just to be safe.
UPDATE: 2015/04/02 5:33pm PDT BY
OTA on April 7th
T-Mobile has confirmed the OTA will start on April 7th. The update docs are already live on the T-Mobile site if you want to have a look.
https://twitter.com/askdes/status/583684027887923201
UPDATE: 2015/04/26 12:49am PDT BY
Strangely, the T-Mobile support page says that the 5.0.1 over-the-air update has been halted.
The Android 5.0.1 / Software D85120b update is paused. We want to make sure we’re providing quality updates to enhance your experience. This update provides features and improvements:
T-Mobile isn't saying exactly what the problem is, but comments on this post (originally published on April 2nd) say that users are encountering a variety of performance, stability, and battery life issues.