Download a VR app of your choice, or find a 360 video on YouTube with the VR setting enabled. You can tell if a video has a VR setting by looking at the bottom of the video. It will have a goggle-shaped icon which can be enabled.
Turn on the app of your choice and click the goggle-shaped icon.
Unlock the hatch at the front of your VR headset and place your phone into it, screen facing you.
Adjust the straps on the top and sides of the headset until they properly fit your head.
Use your app or video!