One of the best values in landscaping is a truck load of stones of varying size. You can usually get three or four tons for around $100 depending on your local economy and availability of materials. In the mid-west there is an abundance of "river rock", usually rounded organic shapes rocks that were pushed around by the glaciers during the last ice age. Typically any quarry in the mid west that offers sand and gravel has a pile or two of stone to offer too. I'm not familiar with the types of stone available in the South, New England, the North West or the South West, but the general specifications for this project are the same. You want a number of stones between 12 and 24 inches wide and between 6 and 14 inches high of varying depths. The easiest way to come by these is to have a load of randomly sized rocks delivered. I say random as opposed to screened by size as the random lot will provide materials for other project's I'll be discussing. You'll have a pile of rocks in the yard for the summer but the kids will love you and some of your neighbors may even confess to a little envy. Either way you go, a small sized load (often you can pick your own if you can transport them yourself), or a large random selection should produce a nice set of stones suitable for a simple staircase.
You'll want a sturdy shovel (the kind with the rounded end, not a spade), a good pair of work gloves (the new latex coated elastic kind I find have a superior grip and wear to leather work gloves) and some common sense: always lift with your knees, not your back or groin. Optionally, you may want a stone boat and or pry bar for transporting and manipulating larger stones. In my case, I made a simple stone boat out of a discarded car tire and a tow chain attached to my garden tractor. The tire slid harmlessly over my lawn and driveway and was slick enough that I was able to pull a 100 lb. stone with my tractor.
Using natural, as opposed to manufactured, stones gives a very rich and pleasing look and feel to your work. Manufactured blocks provide precise edges and can have their appeal, but a really professional look can be achieved with stone. Ideally, you're going to pick stones that have one or more large flat sides. This will be the run of the step that stone will become. Pick, if you can, stone colors that contrast with or compliment the color of your house and other surroundings. Picking colors that are too similar to your house can create a bland appearance where the stone work fails to stand out against the backdrop of the house. The same goes if you are installing your stair near a deck or gazebo. You want stone color that won't clash or disappear. Light colored sand stone is a value shift, color wise, from browns and tans. Red granite is a value shift from redwood, and white or grey granite a value shift away from cooler grays and blues. Red granite with a blue house creates a bold contrast which may possibly be what you are looking for, but keep in mind the curb appeal (even in the back yard) of your house is derived from what most people will think about it, so choose wisely. My personal favorite is the white to light grey granite as it goes with the most colors and settings, whether near a pond, a flower bed or an old half of an oak whiskey barrel. What you get may be random as well, which only adds character and interest. In the example shown here, I have availed myself of limestone, granite and other igneous rocks that have good density and resist splitting. The limestone probably won't hold up as long as the others, but it was a freebie (pushed into my yard when the house next door was built). Pick what you find pleasing if all else fails.
Any part of your yard with a good amount of slope to it will do. Generally something with a 4% or 6% grade (about 30 to 45 degrees if viewed from the side) will provide a good location. But that's just the mechanical aspect of it. Aesthetically, you want the stair to serve as a transition between one area and another, leading someone through your garden or other landscaping through a particular point in space. This funnelling of foot traffic makes planning the rest of your landscaping much easier as you now have a set of focal points to plan around: the view from the bottom of the stair, the view down the stair, and the view to either side of the stair while ascending or descending. In my case, I had a foot path through the dense honey suckle shrubs that have taken over a portion of my lot. Most fortuitously, there was also a geed deal of slope along the path, which made the ideal spot for a stair. Take a look at some other examples.
If your site is ideal as mine, you may be able to get away with minimal work. All I had to do was carve out a bit of ground for each step, starting wit the bottom step and working my way up, and then use some fill dirt packed in around the edges. You may need to adjust your stone several times to get the run (the flat top) sufficiently level. Use gravel or earth which is free of organic matter such as weeds and sticks for leveling by throwing a handful at a time under the stone and checking the level each time. Stand and jump on each step as you go to help seat it and to determine if you have a snug enough fit. You don't want them to wobble any at all as it could lead to injury. After it is seated in place, fill around the edges with more dirt and use the handle of your shovel, inverted, to pack earth in around the edges.
To minimize shifting of your stones over time, you may want to dig them into the ground deeper, or if this is an integral part of other landscaping where no shifting and settling can be tolerated, consider digging twice as deep as you need and start with a bed of well packed gravel and other smaller stones. The Romans built their roads to last and typically put the largest monolithic stones on the bottom and then progressed to smaller, tightly packed stones ending with cobble stones. Laying a foundation of other large stones will minimize shifting from freeze and thaw over the years. In my case, it's a foot path in the woods and occasionally shifting (and maintenance) is acceptable to me, so one layer of large stepping stones suits my needs.
To wrap it up, I'll be adding some Sweet Woodruff as a ground cover on either side of the stair owing to the shady and dry soil in this area. When completed, it should be a very pleasant surprise for anyone strolling through my wood.