The short answer is - practice! :roll:
This is one of the measures of how good a turner really is - by how nearly perfectly semi-circular his/her beads and coves are. (Mine very rarely are by the way!)
Sorby do make a special tool for this, with an interchangaeble end for doing various sizes of coves and beads, but it is a bit expensive for what it is, and being a scraper, doesn't leave a great finish from what I have seen. This results in heavy sanding being required, which half the time ends up spoiling the profile. See here for details of the tool
http://www.robert-sorby.co.uk/
Coves are turned using a spindle gouge with a rolling and sweeping motion. Beads can also be done with spinde gouge, but I find them easier to do using a 3/8" beading and parting tool, or even a small skew chisel, although watch out for the dreaded catch!
Have a look here for an enourmous amount of information, including a downloadable beginner's guide to turning.
http://www.turningtools.co.uk/
The guide is here
http://www.turningtools.co.uk/wtintro/wtintro.html
Also here for some videos - particularly beads and coves
http://www.turningtools.co.uk/videos/videos.html
I hope that this helps.
Regards
Gary