Water in a manner that doesn't wash out the seeds or seedlings. Gentle dribbling or bottom soaks in a pan of tepid water work well.
Soil that looks muddy probably is. Add some more drainage slits to the base of the flat and/or reduce the amount of water you provide.
Soil that looks and feels dry needs water. It is crucial that you do not let flats dry out completely after germination, the seedlings will wilt and die. You're there so you have to judge your flats with your eyes or feel for moisture. Adjust as needed.