1st, use focal length about 30-50mm like that. try to use Aperture priority mode, set to the lowest F-number u can go. if u're using the 18-55 kit lens, shud be around 4 or 5.

get someone to pose like that for you. get as close as you can (get nearer, dun change ur focal length) until the left hand is almostttt out of the frame. compose ur shot, n focus on the right arm. that way, the left hand will be blurred. this is caused by the distance of ur focus point (the right arm) and the left hand.

actually you can make the left hand more blur by using 18mm focal length because of two reasons: 1) 18mm will exaggerate the length between ur focus point and the left hand, and 2) u can use a bigger aperture like F3.5. but then this will make the left hand look huge~