tez is completely right there with his/her advice, however I can give you some easier and much proven advice. get a length of torus skirting the length required to fix in one piece also considering the mitres, then simply do the following:-
Take two chairs or two objects steady enough to support the skirting at each end, (advisable to do this in a long warm room) and if you are using MDF skirting simply place a weight of about 5 kilos in the middle of the skirting and dampen the back of the skirting with hot water and leave a hot bowl of water underneath and change it every hour for at least five hours, this will slowly bow the skirting into enough of a shape for you to mould into your bay.
if you are using real wood skirting follow the above but just keep the wood damp but not wet
I know I know I am a genius and you are very welcome. :-)