Each game is different, and each platform is different, so the process for recording yourself playing your game varies widely. Metafy has created a guide for students to find instructions that apply to their gaming platform.
You can find the Recording Instructions here: Recording Instructions.
Additional Tips
Try to minimize dead time on your recordings. Ideally you should start recording when the match starts and stop when it ends. Alternately, you can trim your recording to just the match you’re submitting.
If you send a single video with multiple matches in it, use the Review Editor to add timestamped comments to indicate where each match starts in that video.