It's hard to go wrong with creating a raised bed to plant your perennials in. To begin with, the vast majority of perennials appreciate the good drainage a raised planting area facilitates. Plus, installing a raised border allows the area to be filled with loose, fertile topsoil. The border also frames and defines the area. A special type of lumber called "landscape timber" is an inexpensive and easy way to build a solid, stable and attractive border.
Timbers are large pieces of wood that are generally as wide as they are tall. Common sizes are 4-by-4 inches and 6-by-6 inches. A specialized "landscape timber" is usually about 3-by-3 inches, but has rounded sides that make a more decorative border than the conventional timbers with their sharp right angles. The smaller size is also much easier to work with -- installation of big timbers is difficult because they are so heavy. Timbers may can be made from any type of wood, but it is best to go with pressure-treated wood or something naturally rot-resistant, like redwood or cedar.
The big drawback with using timbers as a border is that you are limited to working with straight lines. On the other hand, the clean, crisp lines have a nice aesthetic of their own and can facilitate lawn maintenance. For example, it is easy to use a weed-wacker against the straight, solid edge of timbers compared to a natural stone border with all its tiny crevices. Square and rectangular shapes are the easiest designs to install, because you won't have to make angled cuts. But if you're armed with the right tools and a little bit of carpentry skill, hexagonal shapes or other, creative forms can be built as well. Mark your layout on the ground with stakes and string to get the lines perfectly straight so you can take measurements. Cut the timbers to the length you need and you're ready for installation.
Any existing vegetation should be removed from the narrow swath where the timbers will be installed. This swath of ground also needs to be dug down so it is perfectly flat and level. It can be difficult to make a perfectly level surface with soil, so you can get it relatively flat with a shovel and then spread an inch or two of pea gravel or paver base which can be easily adjusted to make surface that is level and firm. To keep your timbers from getting dislodged, drill holes every 4 feet or so, and drive a 3/8-inch-by-12-inch piece of rebar through the timber and into the ground.
If you want, you can stack the timbers up to 12 inches tall to give your plants that much more room to grow. If you go higher than this, it's considered a retaining wall and has to be built in a different way in order to be structurally sound and safe. If you're stacking timbers, you need to stagger each course so that every place where two timbers but up against each other coincides with the midpoint of another timber above or below. You will also need to nail the timbers together. Drill a hole every 2 or 3 feet in each timber and drive a giant nail through to secure it to the one below. The nails you need are called "timber spikes" and are available in sizes from 6 to 12 inches, depending on the size of the timbers you're working with.