For rectangular windows, make sure to order window treatment that is wider and taller than your window, i.e. make sure the window treatment can fully cover your window. You can size up as big as the wall space allows.
6-inch rule: add at least 6 inches to the window width for every curtain rod you use when you order online. For valance only or panel only, add at least 6 inches when you order online(one rod); for a full set, add at least 12 inches (two rods). If the wall space is limited, take the wall space(subtract a few inches) as the width when you order online.
Fo detailed instruction, please refer to the measurement guide.
For an arch window, you can treat it as a rectangular window and put the curtain rods over the arch or under the arch.
Or you can estimate the arch width by 1.5 x window width and hang the valance along the arch line (holdback style).
For bay windows, you can simply treat them as three separate rectangular windows when ordering online.