To play the C7 chord, place the index finger on the 1st fret of the bottom A-string. Let the other strings ring open.
C7 Ukulele Chord Explanation
The C7 chord contains the notes C-E-G-Bb.
To build a C7 chord, take the dominant seventh formula (1-3-5-b7) and apply it to the C major scale (C, D, E, F, G, A, B).
Identify the first, third, fifth, and flat seventh scale degrees in the C major scale which are C-E-G-Bb.
Notice how the seventh scale degree found in the chord is lowered a half step as indicated by the "flat" or "b" that precedes the number "b7". In this case, the seventh scale degree of a C major scale – a "B" note is lowered a half step to a "Bb" note to make the chord a dominant seventh chord.
Watch the following video to learn how to play a C7 chord on ukulele.