Raised bed gardens are above ground boxes with no bottoms. They can be built of any material, filled with soil and planted.They are any length, but only wide enough to make the plants within reach for easy care.
Decide where you want your bed. Pick a spot with at least 6 hours of sunshine, more is better.
If you want several beds, they do not have to be in the same area. They do not have to all be in the backyard. You could even make an attractive raised bed in your front yard or beside your house.
Most people use wood planks to make the box. Do not use treated wood. Do not use railroad ties. Cedar or redwood planks are best as they are resistant to rot. You can also use rocks, cinder blocks, bricks, even wood pallets.
The sides of the bed can be built up to any height that suits you. It takes a lot more work to make them tall, but it would save the back later.
After the beds are built, it is time to fill them. If they are 12 inches or less, just put some good soil in them and begin planting. You could use bagged soil from the garden center, or make your own.
If your bed is over 12 inches and you have not turned the soil under the bed, place a layer of newspaper on the grass. Then add more untreated paper, wood or other decomposable materials. Then layer on some compost if you have it, and add your good soil to the last 12 inches.
Now you are ready to plant.