Try using makeup sponges to cover large areas. To add detailed lines, use a outliner tool. It will add a three-dimensional look to your tile. Keep brushes soaking in water so the paint does not dry in them. After cleaning, dry the brush off on paper before dipping it into a color.
Extra water can dilute the paint and make it runny.
Let your design dry for 24 hours. Allow it more time to dry if the paint is thick on the tile. Put the tile in a cold oven. When the preheat light goes off in the oven, begin to time the piece. Let it bake for 35 minutes.