The great director John Ford once introduced himself by saying, "I make westerns," and he certainly did. Ford's pictures set the standard for the genre, and it is a pleasure to report that four of his best will be on the American Cinematheque's big screen at the Aero Theatre in Santa Monica. A double bill of the cavalry epics "Fort Apache" and "She Wore a Yellow Ribbon" will screen tonight, while Sunday has the twin classics "The Searchers" and "Stagecoach." Both nights start at 7:30. If you want more Ford, and who wouldn't, check out the multi-Oscar winning "How Green Was My Valley" on Friday night.