Before you jump into your first art experience check the basic art supply list (18months+).
Start with only one colour and one toy. This is a new experience for your baby, and if you start with too many stimuli it could be too overwhelming. Depending on your child's character you maybe need to give "permission" to cover the toy with paint. Show how to do it and let the little artist do.
After a while you can add a new colour (keep in mind that red, blue and yellow makes brown!!)
Let your child be the artist, as hard it might be, but don't interfere. Swap some colours or toys when you feel it's time.
In the end you can chose the nicest part of the painted paper together and turn it into a framed painting which everyone will be amazed about.
And what to do with the left overs? Well, that's a new art project!