The theme: A simple idea—for Oprah, it was everyone in hats—is all you need to throw a garden party. Your prize dahlias are blooming? It's an event. New patio? Reason enough. The neighbor caught a bluefish? Absolutely. Turn any day into a party: Bastille Day, Labor Day, the dog days of summer.
The look: Once you've chosen a setting—on the lawn, by the pool, under your favorite tree—think about how to serve. A buffet is easiest. For a sit-down meal, get guests to mingle by placing starters, drinks, desserts, and gifts at separate tables where everyone helps themselves. Don't forget umbrellas or canopies for shade. At night, string lights in trees, float candles in bowls of water, hang paper lanterns. A color can hold it all together. Pick one or two colors you love—hot pink, say, or blue and white—and add a touch of it to everything from invitations and table settings to parting gifts.
The invitations: Make a guest list of friends you want to see and one or two people you'd like to meet. Oprah's invitations were handmade, but inexpensive computer calligraphy can be ordered from stationery stores. (You can also order online at sites like ChelseaPaper.com.) Choose paper to match your theme. You might attach a card with ribbon to a flower-seed packet or pretty garden gloves.