Assuming you are using a system that supports video recording, you only need to hit the "Record" button at the bottom of the game window. If there is no "Record" button, try pressing F8. When you do, you should see a Save File dialogue box pop up. This will have you name the resultant video file and pick a location to save it.

Once you played all of the game you wish to record, hit "Stop" (or F8 again) and it will finalize the video. At this time, you should see a browser window automatically pop up and take you to a page where you can submit your video. It has a pre-generated description and title, which you may add to if you wish. Once you are satisfied, you can submit that info and proceed to the next page. Here, you will be able to Browse to the file you saved and select it in order to upload it. Once you submit the upload, you pretty much just need to wait for it to finish.

If you recorded a video more than just a couple of minutes, the file size is probably really large. In this case, I recommend compressing the video before selecting it for upload. You can use Freemake Video Converter to do this (I think David added a link to it either in the game window itself or somewhere along the video submission process... if not, just Google it).

If any of that is confusing, or you need help using Freemake, just reply back and let me know.