To avoid having too much light when you are using long shutter speeds, us small aperture. “You’ll need to decrease your Aperture in a proportional amount to the amount that you lengthen the shutter speed.” You should also decrease your iso. “Choose a low number and you’ll find yourself able to choose longer shutter speeds.” Tring a Neutral Density Filter can help avoid having too much light. “filters cut down the light passing through your lens and into your camera which in turn allows you to use a slower shutter speed.”