Select a fine quality leaf and press it in a large book.
A few days inside a heavy book will usually do the trick.
To make sure the leaf does not decay any further, it must be treated with a sealant.
A clear matt sealant is a good choice for preserving.
Paint away... The shape of the leaf becomes a big part of planning how the subject is painted.
When the painting is complete, I spray with sealant one more time to add that extra protection for the leaf as well as the painting.