Turn on the histogram when setting the exposure manually because the phone screen doesn't accurately reflect what is captured on the SD card.
Exposure usually controlled by manipulation of ISO, aperture and shutter speed. The Spark has a fixed aperture so it's not adjustable. The ISO (sensor sensitivity to light) is best kept at 100 or 200 to avoid the grainy footage associated with higher settings.
The Spark's frame rate is also fixed - at 30fps. To get the subtle degree of motion blur associated with cinematic footage the shutter should be set manually to 1/60 sec (twice the fps). This slow speed usually lets in too much light (overexposed) which is corrected with a darkening neutral density (ND) filter. The histogram pattern is the key in choosing the right filter.
You can always choose auto exposure if all that is a hassle. But then the Spark will make exposure adjustments during your footage which seem unnatural to the viewer - you can edit those segments out later.
There are many many videos on the subject but they all relate those same basic principles.
the recommended shutter speed for videos is double the frame rate. that means if Spark is recording at 30fps then your're shutter speed to be 1/60. that is to achieve the most natural motion blur. in this case you would need some ND filters to limit the light coming into the sensor.
higher shutter speed will "freeze" the motion making the video/image look a bit unnatural. but for actions scenes - like action sports where there is a lot of movement - you can use higher shutter speeds to emphasize some movements/scenes/replays.
Keeping the above statement in mind, I have just one tip for you.
Assign one of the customisable buttons on your RC to AE lock/unlock.
Having done that, let the AC select the exposure on its own depending upon a given scene and lighting (Auto mode).
Hit AE lock before you start moving.
I primarily use ND filters for shooting video. When shooting photographs, you can adjust the shutter speed to get the exposure set correctly. When shooting video, like @dj_dread was saying, you typically want to set your shutter speed to 1/60 for 30fps and then you would try each ND filter and see which gets you closest to correct exposure. Here's a link to an example of ND filters for Spark. There may be other options for ND that others can share. Once you get the exposure as close as possible with the ND filter, then you might need to change the shutter speed slightly to get your exposure correct. You will want to keep the shutter speed as close to 1/60 as you can.