Through painting, give your child the opportunity to express feelings and unleash his imagination.
The emphatic answer is don't.
When young children paint, they express how they feel and do not necessarily depict particular objects. What is important is the act of painting, not the finished product.
When your child shows you her work, and it looks like random splotches to you, refrain from asking, What is it? or What does it mean? Allow your child’s imagination free rein and don't steer her. Provide her with materials and opportunities to paint.
To get started, here are a few practical tips:
The Art Lesson by Tomie dePaola (Putnam Juvenile, 1997)