1. Use a screen-recording program to capture the screen
2. Choose a region of the screen, so that it matches with your Gens window
3. Start recording the window, then start the game
4. When you have a successful run, stop recording, then cut out everything in that video that took place before the good run (if you play for 32 minutes, but start your last run at 30:23, remove the first 30min22sec from the video)
5. Produce the shortened video. Use whatever codec you like, but make sure it doesn't reduce the quality too much (if you only get 3 frames per second, that isn't a good video and you should try again; it's the same if the entire screen is always blurry) and isn't too big a file (use AVI format for higher quality, but if the level is long and your video is bigger than 10-12 MB/Mo, try switching to WMV).
If you ever make one, come back and I'll continue the steps.