Begin with a clean surface. Children are more likely to explore with patterns and notice them if there are less visual distractions. For example, you can set up a box of color-themed manipulatives, such as Legos or blocks, on an empty table.
Use simple objects. The manipulatives should be the same size, so that children are not working with the multiple variables of size, shape and color. Color is the most obvious and simplest variable for preschoolers to use.
Allow children to explore. They may not introduce patterns, but they need time playing with the manipulatives to discover their various properties. After 5 or 10 minutes of exploration, you can model the creation of a pattern for them if they do not do one themselves.
Do rather than say. Create a simple pattern, such as red block, blue block, red block, blue block. Ask the children what they notice about what you did. Allow them to describe the concept of a pattern in their own words and then give them the target vocabulary of pattern.
Ask them to copy your pattern. Some children will only be able to copy the pattern exactly. Others may be able to extend the pattern. They may work individually or in groups to do so. Some children may be able to transfer the notion of a pattern to create their own color combination, such as yellow block, green block. Others may be able to work with more complex patterns, such as red block, yellow block, green block.
Transfer their work from concrete objects to symbolic abstraction. Keep their patterns visible and have them paint what they see. They may not emulate the shape, but they should be able to use the key colors and sequence.