You still need to expose correctly! Exposure is covered in depth in this answer. What you've done is increase one leg of the exposure, by using a longer shutter speed. Now you need to decrease the overall exposure by the same amount to get back to a properly-exposed image.

So for example, if your camera's exposure meter told you to take a picture at 1/60 s, and you carried all the settings over to manual mode but changed the shutter speed to 1/4 s, you have added four stops of light to your exposure and need to remove them.

You can reduce the exposure through in-camera settings:

• lower your ISO, as long as you weren't already shooting at the lowest ISO ("for the best quality"). If you were at 200 ISO, though, you could get one stop back by lowering the ISO to 100.
• use a smaller aperture. If you were already at f/11 to get the most depth of field for your picture, or if you're using a P&S camera without good aperture control, there's not much you can do here. But if you were shooting wide open you should be able to get several stops back.

You can also try to reduce the exposure by decreasing the amount of light coming in to your lens. There are two ways to do this:

• if you're impatient, use a filter. There are several strengths of ND filters, which are rated by how many stops of light they restrict. Polarizing filters typically cut out between 1 and 2 stops of light and also have other uses, so if you just want to buy one filter, a circular polarizer might be a better value than an ND filter.
• shoot when there's less light. Either wait for a cloudy day, or shoot at dawn/dusk, or shoot at night.