for a long time i raised my arm a lil bit, which makes the shoulder stall a lot easier
hold ur arm in i the position this guy does
Shoulder Stall!!practice this part first
after u can do that perfectly, a shoulder stall without ur arm shouldn´t be a problem anymore
try to put the ball on ur shoulder and just keep it there
then, throw it up and try to catch it (now without using ur arms of course)