Design the wall based on the grade of the slope it must retain, the type of soil, water pressure from runoff and other factors affecting its stability. You may need to consult a soil expert for advice. Plan a wall to be at least 12 inches wide at the top, with a base slab or footing at least as wide as 50 percent of the height. This should be at least 2 feet deep or below the frost line, the depth at which ground freezes.
Dig a footing trench the length of the wall with a shovel. Compact the earth with a hand tamper to make sure it is solid. Add an extra base of 6 or more inches of compacted gravel if the soil is sandy or not solid. Pour that footing with concrete and let it cure for about a week. Make the top of the footing about a foot below ground level, depending on the height and style of wall being built.
Pick one of four wall types: gravity, which has a wide base with equal slopes to the top on both sides; semi-gravity, which has less of a slope on the backside but uses steel reinforcing there; cantilever, which has a straight backside with vertical steel reinforcing; and counterfort, which has a straight back but with diagonal braces between the wall back and the slab footing.
Make forms with 2-by-4-inch lumber and oriented strand board (OSB) panels. Cut panels 3 or 4 inches taller than the wall height. Put the smooth side of OSB in to make a smooth surface on the concrete. Nail OSB with a hammer to 2-by-4 rectangles for basic forms, set them in place and brace them with 2-by-4 boards nailed to the forms and to stakes in the ground. Install 2-by-4 separator strips at the top to hold panels apart. Use temporary interior spreaders halfway up the wall if necessary to hold forms at proper angles; these can be removed as concrete is poured.
Coat the insides of the forms with mineral oil so concrete will not adhere. Add any reinforcing bar if necessary; semi-gravity and cantilever walls will need both vertical and horizontal rebar at the bottom of the wall. Check with a level to make sure forms are level and plumb.
Pour concrete into the forms, starting at one end and working to the other. Hire a concrete truck or pour with a wheelbarrow for small walls. Use a 2-by-4 to tamp the concrete firmly into the bottom of the form to be sure it fills all the spaces. Fill the forms to the desired height; remove any internal bracing when concrete reaches that level. Smooth the top of the wall with a mason's flat trowel once the proper height is reached.
Let the wall cure a week, then remove the forms. Put a layer of gravel or perforated drain pipe at the bottom of the back side of the wall for drainage. Backfill with soil, compacting it as you work up. Stop the fill slightly below the top of the wall. Make a slight swale or depression, just behind the top of the wall, to help drain water and prevent it from building up behind the wall.