To play chord straddles you play the outer notes of the chord and leave out the middle note.
To play straddles in the C major chord you play the notes C and G together, then play E and high C together, then G and high E as you are leaving out the C note in between. This continues so on for each chord.
In a 4-note chord you do the same thing, except you play 2 notes of the chord while always leaving a middle note out, then play the other 2 notes, and so on. So for C7 you would play C and Bb, then E and C, then G and E, then Bb and G and back to C and Bb.