Regular windows are usually a snap to buy curtains for. They run in single and double measurements in which both curtain rods and curtains are pre-cut and mass-produced to fit. However, bay windows can be square, angled out, or curved, and are not necessarily of uniform measurements. Don't skimp on window treatments for them, especially as they are often a focal point of the room. If you can't find curtains of the correct measurements, have them custom fitted.
Install the hardware. Your curtains really do not fit the window, they fit the curtain rod or rods.
Measure the length of the curtain rod, or rods if there is more than one. If the rod is curved, take a piece of string and tape it so that it runs along the entire length of the curve. Mark on the string where the curve ends and untape it from the rod. Lay the string flat and straight and measure the length to know the true length of the rod. This number will be the width, which is typically measured in inches for curtains.
Measure the height of the window from the top of the window frame to where you want the curtain to end, whether it's below the window frame or to the floor.
Adjust your measurements. Generally, you don't want a curtain to lay flat against a window, you want some ruching along the width. A safe way to do this is to double the width measurement. Repeat this for all rod measurements.
Purchase curtains in the correct width and length measurements for each separate curtain rod in your window. You may choose to purchase two curtain panels for the longest curtain rod, in which case their combined width should equal the width measurement you were shopping for.