When playing B on the ukulele you can either completely or partially barre your index finger. If using the full finger leave it slightly over the top of the second fret and press down on the rest of the strings, making sure none are muted. A partial barre is using the top of the index finger to play just the E and A strings. On the C string the middle finger plays the third fret, and the ring finger plays the fourth fret of the G string. The last two fingers need to stay as straight up as possible so that they do not interfere with other strings.