[FFmpeg-user] Using ffmpeg to combine video and audio?

[Andrew Valenkov]

Hello, everyone I guess. Never used such "mailing lists" before to communicate with a bunch of people at once.
I am trying to become more computer-savvy, and learn all the digital multimedia trickery. I am also trying my best to preserve select YouTube videos in their original quality.
The situation is such: I have a video file with no audio, and the original audio from that video, both in /Downloads folder. I have searched for the solution, and seem to have found the command for the Terminal. But the closest thing to success I got was "no such command found" or "no such file or directory found".
I am a Mac El Capitan user, and have almost no Terminal experience. In other words, a n00b. And help that results in me succeeding is appreciated.