The hardest part of this is trying to determine where to drive the screws. There are so many combinations of window styles, casings, moldings and hardware that it is impossible to cover every contingency, so the first instruction is to watch a couple of home improvement shows until you see how the framing around a window is arranged. You can try to use a stud detector but with plaster and around a window, you can get all sorts of misleading signals. It is much better if you try to think about where the framing should logically be.
Second, throw away the screws that came with your valance rods and get ones that are the same size/diameter but 2 to 2-1/2 inches long. You want the screws to be long enough to go through at least an inch of lath and plaster and about an inch into the 2x4 framing around the window opening. Position one end piece of the valance hardware where it is over the 2x4 wood framing inside the wall and not over the molding around the window. The instructions that came with the valance should give you some clues on this. Mark the screw hole locations for the hardware with a pencil. Remove the hardware and drill holes on the marked locations. Use a drill bit that is smaller than the screw diameter by about 1/16". Then put the screws into the hardware and screw the screws into the holes. You can use a screwdriver bit in your drill for this, if you have one but be careful of the bit slipping and marking the wall. It is usually better to just get the screw close to tight with the screwdriver bit in the drill and finish the last 1/4" or so with a hand screwdriver for better feel and control. When you have the 3 screws (probably) on one side, then mount the hardware on the other side of the window.
Be careful while you are doing this and don't fall through the window or anything. Use a ladder and not the closest chair if you need to work a little higher. Finish by vacuuming the drilling dust from the area and installing the valance rod. Now call your wife and follow her directions for installing the curtains. You will not be able to get this right. I have not in over 30 years. LOL