Use Windows Media Player to set it up, then it's easy. Make sure you have the latest version. make sure the xbox is on and the wireless is active. Go into Library then Media Sharing.
It should say something like "share my media to" and there'll be a picture of an xbox underneath. Click that and set it to "Allow."
The next part is the part i can't remember to well, but basically, you have to set Media Player to monitor whatever folders the files are in. I think it's library > more options > library tab> monitor folders.
Then, on the xbox, go into the video part and "change video source" to point to your PC. All video files you have in the folders that you've set to shared on media player should appear there.
Not the best explanation in the world, but it's easy to follow when you have the right things up on screen. And i also have no clue how you do it on this new xbox experience thing but assume it works the same.