You do not need to replace soil. You simply add compost and other amendments to the soil you have. And, you should only need to dig one spade depth deep. Here is how I make a new bed.

1. Plan where you want to put plants, and how wide you want the bed to be. It should be at least 3 feet wide, but if the bed is wider than 4 ft it becomes difficult to attend to plants in the back of the bed. Doesn't mean you can't have a wider bed, but you may need to place stepping stones for access. Is the area in sun or shade. Does it border a driveway or a masonry wall? Sun and reflected heat can limit what you plant.
2. Measure the depth of the bed and mark the corners and, intermittently, the edge of the bed with bricks, lawn paint, even flour will work temporarily. Digging should be done when the soil can be easily penetrated, but is not so wet as to stick to the spade. Remove any grass, weeds, and plants you don't want, shaking the soil off the roots and back into the bed. Break up lumps of soil as you go. The volume of soil may seem to increase, but will sink back down in a few weeks.
3. Add at least 2 inches of compost over the soil. It doesn't matter if your soil is sand or clay. Compost helps sand hold water, opens up clay soil, and makes nutrients more available in both. You can also add other AMENDMENTS if your soil is deficient in particular nutrients. Use your spade to half turn the soil so the compost mixes in evenly. A garden rake can also be used.
4. When you are planning your new bed, think about what you want to use as a border. You can use metal or plastic lawn edging, bricks, rock, or treated wood. Check out what is available when you shop for compost, and think about which will look the best, work for your needs, and fit your budget. Once you have the soil amended, place the border along the edge of the bed.
5. Now you can go buy plants. Never buy plants until you have a spot prepared for them. Pay attention to the info on labels. Make sure shrubs won't eventually cover windows; the term dwarf only means smaller than the species. Also, make sure a shrub will fit in the space you have when mature. Expect to have space between plants in a new bed. You can fill in with annuals while the perennials and shrubs are growing.
6. Set your new plants (still in their pots) in the bed where you have envisioned them. If they don't work, rearrangement them to better advantage. Once you are satisfied, start planting. Pull a plant from its pot and check for root binding. Pull loose any roots that curl around the plant. Dig a hole just deep enough for the root ball, set the plant in so its top is level with the bed, and firm the soil back around it. Proceed with the rest of the plants. Then water deeply. Water twice more the first week, unless you have rain. After that water as needed, but remember that all plants need more water the first year they are planted. You should add a mulch over exposed soil. It does not have to be thick, and should not be placed against the stem of any plant. In addition to keeping soil moist, mulch protects soil from heavy rain that can splash soil several feet from where the drops hit. This damages soil structure.

Much easier than digging is this method: Cut the grass short, place cardboard over the grass, cover the cardboard with mulch. Wait several months. The only plants that may come through would be deep rooted ones like dandelions. You can either hit these with a little Round up or pop them out with a shovel and pulling. This technique is not only easier on you, it's better for the soil as all that organic material rots into the soil.

Plant to plant in the fall and next spring. It's too late in the season and too hot now anyhow to put in a brand new garden. A good variety of plants won't even be available in most nurseries and garden centers.