The ink MUST clean out of the screen. If you are in the habit of leaving ink in the image area after the print stroke, this will be an uneven area in the print. It may be hard to get a really bright white with just one print. You will have to do at least two strokes to get a respectable white. For the top color you can use a little more speed.
Good pressure is important here.
You need to drive the ink around any peaks and valleys in the white. You may need to do two strokes on the top to cover the white adequately. There is no underbasing without a flash-cure unit. If you plan to do a lot of flashing make sure the unit is large enough. Some of the smaller units can barely cure a standard garment size image size.