Choice A - Smother the lawn/area using a large sheet of black plastic, a tarp, or a layer of cardboard or plywood. Keep the area covered for a full growing season.
Choice B - Using a sod-cutter, remove the top 2 – 3 inches of grass and soil, then till the cleared area lightly before installing.
Choice C - Cultivate the planting site three times at one week intervals. If perennial weeds are present, cutivate for a full growing season every 2 – 3 weeks and plant in fall.