Mark off the area for your raised flower garden. You can go as small as a 2x2 foot area or as big as 6x6. It's not a good idea to go much larger than that, though, because you won't be able to reach the middle of the flower garden without physically getting into it and possibly hurting the plants.
Till the area, turning up all the soil 12 to 18 inches deep.
Rake the soil until you've broken down any clumps and the area is level.Remove any clumps of clay, rocks, stones, branches or other debris.
Decide on the material you want to use to build the walls of your raised flower garden. Wood is generally the most inexpensive and easy to work with; it's not as formal, however, as brick or landscaping stone. Natural river rock looks fantastic but is pricier. Let the size of your flower bed, the look of the surrounding landscape and your budget help you decide.
Plan to purchase enough material to create walls that are at least 18 inches high. Measure the height of the material, then divide 18 inches by that height to determine how many layers you need for each wall. For example, if you are using an 8 inch high landscaping stone, you can create a 16 inch wall with two layers of brick, or you can go for a third layer to make your wall 24 inches high.
Figure out how much material you need by measuring the perimeter of your raised flower garden. Then divide the perimeter by the length of each piece of material. If the 8-inch-high landscaping stone you're using is 12 inches, or one foot, long, and your perimeter is 16 feet, then you'll need 16 stones per layer. If you're doing two layers, then, you'll need 16 times 2, or 32; for three layers, you'll need another 16 stones.
Lay the first layer out on the perimeter of the bed. If you're working with lumber, you'll want to nail the corners together for stability. If you're working with bricks or stones, simply lay them close together and work them into the soil to make them as level as possible.
Continue adding layers until you reach your ideal height. You can use mortar in between layers with brick or stone to cement the wall together, but it's not completely necessary. Bricks will stay stacked by themselves, and if you fit your stones together as you build the layers, you don't have to have mortar. Lumber walls simply need to be nailed together at the corners.
Fill the raised bed with topsoil mixed in a 1:1 ratio with organic compost. Keep an extra bucket or two of this mix in the garage to add to the bed as the soil settles down.