There are two ways to enable NFC on the LG G6. The easiest way is to just swipe from the top of the screen and select the NFC icon in the quick menu. You should also make sure Bluetooth is turned on here as well.
You can also go to the phone's Settings -> Share & connect and turn on NFC here.
If you hold the center back of the phone to the NFC symbol on one of the earpads of the headphones, you'll see a window pop up asking whether to pair over Bluetooth. Make sure you position the middle of the phone to line up with the middle of the NFC enabled earpad in order for the NFC connection to take place.
Once paired, the LG G6 will then establish an active Bluetooth connection with the Sony MDR-XB950BT Extra Bass Bluetooth Headphones. You need to make sure the Sony MDR-XB950BT Extra Bass Bluetooth Headphones does not have an active Bluetooth connection with some other device or else the LG G6 may not connect.
The LG G6 supports two active Bluetooth connections which means you can keep active Bluetooth connections to two audio speakers like the Sony MDR-XB950BT Extra Bass Bluetooth Headphones and Amazon Tap. By swiping down from the top of the screen, you can select which of the Bluetooth audio devices you want to play to.