The Cm7 is another lone barre chord. Place your index finger across all the strings of the third fret. Leaving a little of the top joint hanging over, be sure to press down on all four strings. All the notes should sound clear, if not make small adjustments until you find the perfect barre position for your finger. You will also have to use a little more thumb pressure as single barre chords make the ukulele hard to hold sometimes.