1. Get a digital video camera.
I got a Flip video camera. It is really affordable, idiot proof, and the software that is built into it helps you edit and upload your videos fairly easily. Of course you could use an old school camera but that is a lot more work, so for the purposes of this guide, I would say "how to post a video battle report from a digital camera"
2. Record a video.
In January I went to a tournament at Tower Games in Minneapolis. It was a lot of fun, and I recorded all of my games using the Flip. I thought I would post them up and wahoo it would be sweet! Well, I am a procrastinator, but also I had no format for how I was going to narrate the video and I didn't have time to do most of the recording because the game time was limited. I had one of my friends record the video but since there were two of us playing games, we didn't get a solid feel for what was going on. Which leads me to my next point.
3. Plan ahead! What part of each turn are you going to take video, who is going to do it, and how long each set should be.
Generally I would say take one after both sides have deployed, then at the end of each player's turn. It takes about 30 seconds to go over all the units on the table quickly and what has died. You can spend more time on any crazy things that have happened, or you can spend less time on a turn if it was super boring (like your Eldar friend's army was all in reserve on the first turn). Generally, it is good theatrics if you record the dice roll at the end of turn 5 to see if the game goes on or not, recap the basics and predict how future turns would have went if the game had not ended (or play out the following turns if they happen).
If your Battle Report looks like the one above, it won't make any sense to anyone viewing it (also I think i edited it wrong and there is a time traveling dreadnought in there that grows his arms back). I will post an actual report from a fake battle later after I mow the lawn and stuff, but right now I am procrastinating mowing the lawn by doing this :)
Make sure that the recorder person knows what they are supposed to do and make sure you have enough time to actually spend recording pieces of the battle. In a tournament setting I would recommend having a dedicated "videographer" because you will be spending too much time on tactics, moving guys around and rolling dice. In a more relaxed setting you can take the time to explain your tactics or thoughts as the battle develops.
(Yes this is all still part of step 3: Plan ahead) When you are speaking think about what you are saying before you say it, filling up the audio with "ummm" uhhh" and "like" makes you sound retarded. It is still stuff that I work on, but it is a good thing to keep in mind, and to keep the flow of the battle report smooth.
Hopefully I can follow my own rules and post up some more sensible battle reports, I really like watching battle vids, and I hope I can post some good ones.