Here is an example of a fluted column you could put on each side of your arch. Basically this can be made from several different trim pieces and then put together, you can buy fluted material and all the trim to make a column like this. I see this design almost exactly, everyday in the houses I work in only they are at the fireplace mantel.

As far as an arch, basically all it is just the top part of a big circle. You could make a template out of cardboard, start with a circle. To make a circle draw a line from each corner of your cardboard both ways and where they meet is your center, from there you can make as large of a compass as you need to make the circle.

You can use an 8' piece of straight 1x2 with a nail in one end and a pencil in the other, nail it in the center of your cardboard and draw your circle. Then on your existing arch find the center by measuring from end to end and the put a straight edge along the bottom and measure down from your center mark. That will give you the height for your circle and you will know where to cut it off on your cardboard circle. From this you can begin to make whatever arch you want to put up.