What you want to do is develop a pristine environment, especially for nesting. Recently, the tragic frost in Mexico killed over half of the monarch butterflies. You and I must help them if they're not to become extinct. To provide a suitable environment, you'll need food, water, shelter and a place to reproduce. Plant nectar-producing flowers, such as zinnias, marigolds, yarrow, lavender, lantana, butterfly bush and cosmos in a sunny, sheltered part of your garden.
Water can come in a small, shallow dish with a rocky or sandy bottom.
Parsley and carrots are great for butterflies to lay their larvae on. So is cabbage, passion vine and pearly everlasting. Though I'm not sure monarchs breed here rather than in Mexico, other butterflies will appreciate the help. Once you spot the caterpillars, leave them on the plant and watch for chrysalises, but don't touch them. And, come on, you can give a couple of vegetables to others.