Lay down tarps on the floor, and cover anything that you do not wish to be ruined. Have the children change into play clothes or old clothes or smocks. Set up your painting materials.
Show the children the paint, and inform them that this paint will not wash off once it is dry. Talk to the children about the best way to keep permanent paint from ruining their clothes. Tell the children not to allow the paint to dry on the brush, because dry paint will ruin the brush. Show the children that placing used brushes a jar of water will prevent the paint from drying in the brush until the paint is washed off. For older children, much of this should be common sense, and you may only need to touch on these ground rules briefly.
Assume that the children are mature enough to understand the basics of painting--dipping the paintbrush in paint and applying the paint to the painting surface. If they do not understand the basics of painting a picture, then you should not begin with acrylic paint.
Pass out a canvas board to each child. Encourage the children to paint whatever they wish, but give helpful suggestions to those who are not sure. Examples of potential subjects include a flower, a family pet or an illustration of a book or story. Older children may wish to draw their subject on the canvas board before beginning to paint. Encourage the children to cover the entire surface of the canvas board with color.
Monitor painting progress closely. Watch for any paint brushes that may be left out to dry.
Stand at the sink at the end of the painting session and show them the correct way to clean all the paint from the brushes. Assume that you will need to check the brushes for any excess before they are dry, if your children are young. If the children are older, remind them they will be expected to replace any brushes they ruin.
Show the children the correct place to allow their paintings and palettes to dry. This should be someplace where the canvas boards will sit undisturbed. If you are in a classroom, there should be a cabinet or a counter where the paintings may be safely stored. If you are at home, a garage, porch or basement should do.
Help the children pick up the rest of their painting materials. Show them where the jars are stored, and ask them to put away whatever they can.