It depends on what you consider easy! You should kill the St. Augustinegrass first, or you’ll never be rid of it and you’ll wind up with a mix of grasses. If you don’t like St. Augustinegrass alone, you’ll like a mix even less. Spraying with a nonselective herbicide such as glyphosate (Roundup) is the standard way to do this. (Be sure that the grass is well watered before you spray it; glyphosate works better on actively growing plants.) Till up the dead St. Augustinegrass and smooth the area before you establish the fescue. Fescue is most commonly started from seed, but fescue sod is available and provides faster results, though at a higher cost than seeding. An option that avoids tilling is to kill the St. Augustinegrass, then rent a slit seeder to seed fescue right into the dead grass. The slit seeder places seeds in rows several inches apart. Although this saves the tilling step, and the fescue will grow, full coverage will take longer with the slit-seeded fescue than with broadcast fescue seed.