It's an optical illusion of course: the astragal is half a cylinder and our job is to infer the rest. As with all mouldings, the carving is built up cut by cut and... voilà! There it is.
Try to be as exact and neat as possible. It always takes a while to get your eye and hand in with this sort of work, so plenty of practice first.
In the download I've given the tools that I used but do try with similar ones to see the different effect or a different pitch. In particular, depending on how you run the initial, spiralling ribbon cuts, you may find you need a narrower chisel to clean off what will be the top of the stick. If you don't have a chisel, use a flat gouge.