Please remember that all activities are to INTRODUCE, not MASTER Kindergarten Common Core Standards. Every child moves at their own pace.
Be patient with yourself and your preschooler as you introduce new concepts. Some days they will embrace your ideas, and others, not so much. It is all age appropriate and exploration is the key to success.
Sometimes a simple question such as, does it look right, will help your preschooler reassess placement.
In Kindergarten, children will compose simple shapes to form larger shapes. One example would be to merge two triangles to make a rectangle. Children will also model shapes in the world by building shapes from components.
As your child explores these concepts, have them analyze if it "looks right".
Remember, your preschooler may or may not be ready to master this skill, but give them time to play with shapes and manipulate them into larger components.