Window shutters were originally for the purpose of protecting fragile windows against storms, but with the advent of stronger and more durable types of window glass, shutters have evolved into primarily decorative exterior accents. Real shutters are hinged so they can swing over the windows, while decorative shutters are simply affixed to the exterior wall on each side of the window.
Traditional exterior window shutters are made of painted wood. If you are interested in a traditional look and feel, you can make inexpensive shutters out of any wood you have around. Softwoods such as cedar and spruce are best because they weigh less than hardwoods and are more resistant to rot and deterioration when used for exterior purposes. If you are simply looking for functional, inexpensive shutters, you can also use metal or plastic. Be aware that plastic shutters will make your house look cheap.
Traditional shutters were constructed of louvres that could be moved up and down. The reason for making them mobile was so they would shed the rain when they were in an open or a closed position. When the shutters were closed, the angle of the louvres was reversed so they were still pointing down and water would drip off to the outside. You can build shutters like this, but they are quite labour-intensive. You can also build shutters with fixed louvres, which is just as good if your shutters are decorative and won't be opened and closed. You can also build shutters out of solid planks.
The process of mounting a shutter differs depending on whether it is a mobile, functional shutter or a decorative shutter. Functional shutters need to be hinged along the edge closest to the window so they can be swung shut to cover the window. These shutters will also need a hook on the shutter and a corresponding eyelet set into the house wall so the shutters can be held open and won't swing back and forth. Shutters that are purely decorative can simply be mounted to the exterior wall of the house with screws.
Search your local architectural salvage yards, used building supply stores and junkyards for old shutters. If you are very lucky, you may find some that are the right size that you can clean up, repaint and use. Even if you find shutters that are larger than your windows, depending on their design you may be able to cut them down and use them. If you can find used shutters at a low price, this can save you a lot of labour and enable you to benefit from the aged patina of antique architectural accents.