OK - now that we've seen how to make chords, let's do something with them. We'll start out by strumming on the quarter notes, then move on to eighth notes.
There are, of course, thousands of ways to strum a guitar. How you choose to strum a particular song is really up to you; you may even play a song a certain way one day only to alter it drastically the next day. The possibilities are infinite, but these few examples should be enough to get you started.
In order to teach strumming, I'm going to use a shorthand method for the music. Instead of notating music using notes and lines, I will just represent the time that you should strum the guitar using a forward slash "/" character, and then list the direction of the pick using "D" for a down-stroke, or starting from the 6th string and strumming towards the 1st string, and "U" for an up-stroke, or starting from the 1st string and strumming towards the 6th string.
The following illustration shows four measures with I-vi-IV-V as the pattern, playing only on the quarter notes, or once for each beat. The letter D indicates a downward stroke of the pick, or starting from the 6th string and strumming towards the 1st string. This gives us a basic building block to work on...
Most new guitar players only use a down-stroke strumming pattern, and you should get used to strumming both directions as early as possible, as shown in this example...
Strumming both directions is essential to playing better, and allows you to do more with the rhythm of a song.