You'll notice you're given no indication of how to sing the lyrics or how to strum the chords, so you will either need to know the song very well or have a recording of it handy so you can get the rhythm right.
As colored icons come down the note "highway" on the game's screen, the player must then hold the appropriately colored fret button(s) down and hit the strum bar in time with the music.
Although taking these lessons online offers many benefits, it might still be a good idea to take a lesson or two in person to learn the basics of how to hold a guitar and how to strum.
The tab book Grateful Dead Favorites to Strum and Sing is also available at Amazon.com for those looking to jump straight into the songs they know and love from this seminal band.
Demonstrations: No matter how great the video, there is nothing like having a live person there to show you exactly how to hold the guitar and how to strum the chords.