Well it depnds on what amount of cost you want to put in, and how are you making the roundabout, and are you going to put something on it, and essentially wht budgets you have. Where I live, we almost always have some statues or soemthing put on them.

They are made with essentially make shift compasses where I live. This is what is done for a roundabout of radius, lets say in ball park of 5-10 m. Initially marking is done (this is simply done by measuring out a tape of desired radius, and holding wwhen end at center, and other person revolvinng around it, and laying down something - could be simple as white chalk, or use jackhammer to make actual indents to ground, or lay down some bricks) This will serve as guide for curve, and any further curved elements could be made by just maintaining a constant distance from this circumference. Then the interior is usually dug to lay a foundation for whatever you put on it (could be as simple as excavating, and just plastering, but that is still expensive, often manually digging and then laying a firm dirt layer is enough). then it is covered with something (could be custom designed pizza slice shaped stone or marble wedges, but usually just bricks or cinderblocks). While making, we do not really make a great curved edge, and just make a n-gon, where n can be 20 or 50 or 100. This is simpler because straight edged bricks are cheaps, and you can lay them vertical, filling the gaps with plaster. The depending on your budget, you could get great finishing, again done with applying plaster, but this time, excess is applied, and some is removed, so we get a curved edge. Now you mostly have the exterior ready. On the top, you could have marble or stone, which can be grinded to match the curved edge, or it could be a thin plaster layer, which will take the shape of boundary.