Installing a stepping stone path that remains even and attractive for many years is mostly a matter of planning and preparation. You want stones that are large enough but not too large. You want them spaced so that they accommodate your stride. And you want them set properly, so that they won't sink or buckle.
One common problem is stones that are too small, so that you practically have to go on your tippy toes to navigate them. The stones should be wide enough that you don't have to walk a tightrope, putting one foot directly in front of the other, and easily accommodate your foot. A good way to determine the right size is stand with your feet about shoulder-width apart. The right stone is about as wide as the distance between the outside of each foot, and two or three inches longer than your shoe. Stones that are about an inch-and-a-half thick last forever, but won't be so heavy as to be unmanageable when you are setting them.
Setting your stones two feet apart (that is, two feet between the center of each stone, not two feet between the edges) accommodates the stride of the average adult male. Using this as a starting point, set out cardboard squares or newspaper sections to lay out a mock path so you can determine the proper spacing, as well as how many stones you need.
Place your stones on top of the grass or dirt exactly as you want them set. Work on one stone at a time and use an edger or trenching shovel to cut the shape of the stone into the ground. Remove the stone and then dig out the area to a depth of two inches (for a 1.5-inch thick stone). Fill the bottom of the hole with one inch of sand, level the sand, and then put the stepping stone in place. Repeat the process until all of your stones are set.
Setting your stepping stones this way is obviously quite a bit more labor than simply dropping them on top of the ground, but it's worth the extra effort. Setting them in sand makes it easier to level the stones with the ground, prevents wobbling, and makes them less prone to tilting, which can occur from the cycle of the ground freezing and thawing during winter. Installing the stones so that they sit slightly above ground level keeps them from "shrinking" over time as dirt, debris, and lawn clippings creep in from the edges. You might sweat a little more putting it in, but you will be glad that you did five years from now, when your stepping stone path looks just as good as the day you completed it.