Probably the easiest way to make a circle or other shape is to use a long water hose. Lay it in the shape that you want. Once you are satisfied with the shape you have you can mark it with spray paint or by using small stakes and a string.
To remove sod (grass) I think there is a speciality tool called a sod cutter although there are probably other ways to do it too. If you cut it out manually you will need to mow the grass at the lowest setting so the grass will be very short.
As for soil, it depends... You may be able to use what you have left behind. If you need to add to it then see if there is a supplier/nursery in your area that sells soil by the yard and ask them what they recommend for you what you want to plant.
Weed free requires a constant thick layer of mulch.
I hope this has been of some help. Since you are in S.A. you might check with the folk at Calloway's or Rainbow Gardens.

Creekwood
Texas 8b

I took a workshop once here in SA. He said to dig out the soil 18" to 24" and throw it away. (He did not say where to throw it. LOL)
Then fill the area with a good landscape mix. I really believe the best landscape mix available is at Gardenville. They are on Evans Rd. I have never gotten anything but an excellent product from them. They make it right there. It has compost etc. in it. You can have them deliver or go out and fill 5 gallon buckets yourself. I have been using their mix for 20 years.
Once you install your plants, besure and spread compost around them. This will conserve water and keep your plants from baking in our heat.

This sounds like a lot of effort. But the very first bed I did this way has never had a plant die for 20 years.

I used a can of neon garden spray paint I purchase in walmart to make a horseshoe shaped bed last year. I saw this idea on a garden show (don't remember which one,) a few years ago. I guess you can use regular neon spray paint as you're going to be removing that part of the sod anyway. To be extra careful about the shape I first laid out an old garden hose, and then sprayed around that. Worked like a charm.
I then removed the sod, with a perennial spade, breaking up the area into small sections, then cutting into the grass in small strips. When I had one area done I took a fork and lifted each strip and put it in a wheel barrel to put in the compost pile. Whatever you do, don't stick the spade straight down. Slant the spade at an angle otherwise you'll remove not just the grass but the topsoil too. Once all the sod was removed I double dug the bed again in sections. Makes it much easier. I then incorporated 2 large bales of peat moss and about a dozen bags of chopped leaves. I did my bed last fall. I then added a mixed dry fertilizer, and a bag of triple phosphate. I didn't add any cow manure because the soil in the site was good to begin with (used to be a pasture) and the other amendments was all that was needed.
In this bed I planted a dozen peonies, siberian irisis, and dozens of assorted perennials, like poppies, jacobs ladder, balloon flower, verbascum, lychinis, ornamental onions, tulips, spanish and english bluebells, etc. I left 3 spots (1 center back and 2 foward, on either side) to plant bareroot roses in the spring. To remember where to put the roses I places 3 large rocks in each area. I still have to order the roses for this site.
As an aside and a sign that I made the right shapped bed for the area, while double digging the bed I dug up two horseshoes. One for a drafthorse, the horseshoe was hugh, and the other for a regular sized horse. I plan to incorporate them as ornaments for the bed.
One word of caution. Take it easy. I was in such a hurry worrying about getting all the peonies in the bed before they die that I pulled my back out and was out of commission for a week! This might not be the easiest way to make a bed but it is by far the best. The tilth of the soil will always be fluffy and friable, unlike lasagna style beds, which tend to pack down after a while. But if you're in a hurry you can use that method, as I have in the past, to save time. Hope this posting helps and good luck.

I personally do not like sod cutters. All you need is newspaper that you soak in water until moist, or lay the newspaper directly onto the grass and wet thoroughly. You can either us compost, peat moss, leaves, or put topsoil directly onto the paper and continue process until desired height is reached. You can start this in the fall and have a wonderful sight for a new bed in the spring, or you can plant right away. Sod cutting is back-breaking work, and not necessary. Patricia Lanza has several fantastic books on 'Lasagna Gardening", that are a tremendous tool.