This free valance plan from hammerzone.com is written up to be a fluorescent light valance. However, you could apply the same simple design to build a valance for a window. In fact, that is probably the more common use.
The material list is short and could be altered from the existing wood plan. In the original plan, they recommend stock lumber with an edge detail that you rout. If you have a router, with some nice profile bits, this is fine. If you don’t have a router, don’t worry. Just go with some stock molding or trim. There is a great book titled “Trim Idea Book“ that can usually be purchased at home depot and lowe’s that shows different profiles that you can create with stock molding. If it is there, you might want to take a peek while making your selections.
You will also notice that they utilize a biscuit to join the trim. This can be done with glue and nails, hardware, or a wood corner block.
This is an inexpensive woodworking / home improvement project that has a huge impact. Good luck.