First decide what type of a mount you want to do.

Width  - Starting from the left top of the window, take a horizontal measurement from left to right of the window, including any mounted hardware to be used or covered.

Length  - For the length dimension, start from the top of the rod or bracket above the window and measure down to the desired length of the topper.

For accuracy, do the length measurement at the left, right and center of the window.

Tip: To determine the length of material needed for a topper, you need to measure your window and then triple that number. For example, if your window measured 42 inches (42x3=126) you would need material 126 inches long.