Place your ring finger on the third fret on the A string, which is the second-to-lowest string on your guitar. This will create a C note.
Put your middle finger on the second fret on the D string for an E note. The D string is the third-to-lowest string.
Hold your pinkie finger on the third fret on the G string -- the third-to-highest -- for the B-flat note.
Press your index finger down on the first fret on the B string, which is second-to-highest on your guitar. This will make another C note.
Play the open high E string to complete the C7 chord. Strum the guitar without hitting the low E string.