You can simply enter the contest by filling out the registration form on the official Lumia Facebook page. More entries can be generated by inviting your Facebook friends to fill out the form.
The contest ends on March 31 and a few days later five winners will be picked to receive a Lumia 830. There's no word if this is an unlocked version of the phone but hey, at least it's free. Just a reminder: It has a 5-inch display, a 1.2 Ghz quad-core Qualcomm Snapdragon 400 processor, with 1GB of RAM and 16GB of internal storage. It also has a 10MP rear camera.
Source: Microsoft Lumia US (Facebook)