This challenge is for uploading one of your multiplayer matches to either the server or to youtube. You do this by doing the following...
Go to Multiplayer, from there you select replays. Assuming you've already played a multiplayer match (Be it local, player or ranked match) you should see tiles of different matches in your replay browser. Simply press X on it, and upload it to either the match server or to youtube for your trophy! You'll have to enter a tag but that doesn't really matter.
If you haven't saved a replay, you'll have to play a multiplayer match of some sort. Simply go in to local match or head online if you don't have a second controller and finish a match. After it's over, you'll see a summary of any challenge progress and then you'll have a list of options. Select "save replay" and then refer to the first portion of the solution.