First, I am assuming you have already downloaded the game (http://gamelab.mit.edu/games/a-slower-speed-of-light/). In order to record a run, I would recommend downloading XSplit Live Broadcaster (found at https://www.xsplit.com/ -- make sure you download the Broadcaster and not the Gamecaster). Complete the installation, then open the application. In the top bar, between "Broadcast" and "Playout", select "Record" --> "Local Recording". Once this is pressed, XSplit will automatically record any game played on your computer. For now, the screen should be black. Open A Slower Speed of Light. It will take up the entire screen, but XSplit will still be recording. Now, complete as many runs as you want. The recording will not time out, in my experience, for at least a few hours. Once you have a run you are satisfied with, exit A Slower Speed of Light (closing the window is fine) and go back to XSplit. Under the "Record" tab, press "Stop local recording". You will now have a video file in the Videos folder on your PC (or in Finder on Mac). This video is a screen recording of just the game window. Now, using a video editing tool, trim the entire video except the desired run (only ever upload one at once). If you do not have a video editing tool, contact me at origamiman314@gmail.com for a copy of Windows Movie Maker. Lastly, on a YouTube channel, upload your run video. Now go to the speedrun.com page for this game, select "Submit Run", and follow the instructions there. If all went well, your run should be verified soon and you will appear on the leaderboard!

Happy running!