Stake out the area to be used for the patio and use a yard-long level on each of the four sides, determining the high end and the low end of the patio. Do the same on each diagonal. Then you get a flat shovel and shovel out the high end and just scrape off the grass on the low end. Keep measuring with the level until you have a level field.
Make it fall away from the house at least 1/4 inch per foot or more if the existing slopes will permit.