Here is a bit of advice from an art teacher who has had years of experience working with little kids.
Remember that all kids are different and you have to try different approaches to each one of them. Don't push them hard. The main idea is to have a fun time with your child and if he learns something at the end, look at it as a huge bonus! Sometimes even mixing different colors together into a big brown spot could excite a little kid, so enjoy it with him. Praise him a lot! Always have the patience to discuss what it is that your child has made. And you definitely don't have to be an artist to make these "masterpieces" with your kid. Maybe you'll learn something too.
If you have older siblings try to involve them too. They can do the same projects or more challenging ones if they want.
Before you begin - some cleaning ideas.