Lay out the line of your flower bed by using a string and stakes to make straight lines and a garden hose to create curves. These lines are major features of your garden design, so spend enough time to get them right. Consider the yard as whole, not just individual beds. Often it's easiest to see the lawn as a green pond you're creating, a shape that influences everything around it.
Cut along the lines you've defined, using a shovel. First make a shallow, slanting cut on one side of the string or the hose, then go back and cut from the opposite side, freeing a strip of sod about 3 inches wide. Remove the string or hose and the sod, then remove any remaining sod around your flower bed.
Cut the edge again with the shovel, making a trench about 6 inches deep and 4 or 5 inches wide. This acts as a barrier to the lawn grass but will need to be repeated yearly, perhaps more often. You may also install a commercial lawn edging.