The basic idea is to throw slices of fruit into a big pitcher with some sugar, a splash of brandy, and a bottle of wine, chill it for an hour or so, add a little seltzer for effervescence and pour into ice-filled glasses. Orange, lemon, and apple slices are the classic fruit options, but it's fun to mix it up with plums, cherries, and berries when you're using a red-wine base or kiwi, pears, peaches, and apricots when you're in a lighter, whiter mood.
You can also alter the flavor through your choice of liqueur. Supplement or replace the brandy with an orange-flavored kicker like Curaçao or Grand Marnier, or bring out the flavor of your chosen vegetation with applejack, kirsch, or peach or apricot brandy. Adjust the amounts of sugar and brandy to make your sangria sweet or dry, soft or potent, and muddle some of the fruit if you want a more summery flavor. A branch of mint is a nice touch. Or if you don't want the fun of stirring up your own, Zarzuela (2000 Hyde at Union) and Ramblas (557 Valencia at 17th St.) offer admirable examples.