There are several different ways to play it, but I will demonstrate the easiest way in the following post. The C7 Chord is found by first locating the 1, 3 and 5 positions (C, E and G) of the C Major Scale: C D E F G A B C, and then adding a flat 7 note. To locate the flat 7 note, we first find the 7th note of the C Major scale (B) and then go back a 1/2 step to the BFlat.
So the notes of the C7 Chord are: C, E, G and Bb.
Here is a key that will help you read the chart:
C7 Guitar Chord Video
The video below will further demonstrate how to correctly play the C7 Chord. Enjoy!