You can look at some Bay window curtain sets that will help you assemble the curtains together - http://www.swagsgalore.com/baywicurod5.html
It can get tricky that why I like these sets, here is another - http://www.countrycurtains.com/category/300+rods+and+hardware/320+bay+window+rods.do?code=WG020061&kwid=
Did you want design ideas or the technical how-to?
There are over 5,000 ideas here on Houzz. http://www.houzz.com/curtains-for-bay-windows/p/16 I hope this helps.
One good way is to use sheers on spring rods top and bottom to keep the sheers up against the windows, then place a long ornate rod on the wall just above the front of the bays (room-side) and hang drapes or linen curtains there and tie back. This gives the appearance of a deeper window with a welcome feel.
Do you want draperies or just valances?