There are many ways and various materials used to trim a circle top window. First you will want to decide if the window trim will be painted or stained. This knowledge will let you make the best choice of materials.
If your trim will be painted your options are wood, a composite or a flexible rubber trim. Most circle top trims can be ordered to fit the arch. However, flexible rubber trims allow you to fit the trim to your windows size. If you use wood, the trim will most likely be finger jointed
. This is fine and will be concealed if painted. If you stain it the joints will show through and you will have a slight color variation from piece to piece. A composite material would again be a paint grade trim. These trims are normally smooth and take paint well.
If you have a wood stained window, you will need to get a wood trim to match. The best way is to get the circle top trim is from the window manufacturer. This way you will get the correct radius. Not all circle tops are a true half circle radius. Be sure to determine this before you purchase your curved window molding. Common wood trims would be made from oak, pine, or cherry. Be sure to ask if there will be finger joints or made from a laminated veneer. The laminated veneer will give a more uniform color when stained.
Adding a little flair to the window will help with it's appearance. Circle top trim ideas are endless. You can add crowns, coves, shelves, plinth blocks and more creating a rich looking window. Take some time put some thought into it, you'll be happy with the finished project if you do.