Draw or trace a clipart leaf pattern onto some paper or card stock and cut it out with scissors.
Place your paper stencil on your doormat and use a paint brush and white paint to stipple the paint on. Work up and down, with a small amount of paint at a time to avoid overloading the paint and having it seep under the stencil.
Move the leaf stencil around to create a random pattern of leaves on your mat, including some coming off of the edge of the mat. Allow the paint to dry fully before using.