Width...
Measure the inside width of the window from sheetrock to sheetrock (using a good metal tape measure for accuracy). Make three measurements, top, middle and bottom.
Next...

For INSIDE MOUNT...
Use the smallest of the three measurements for the width you will order.
The factory will make the proper deductions for best fit.

For OUTSIDE MOUNT...
Add however many inches you want to the measurement to suit your needs for best light coverage. This is the Width you will order. The factory makes no deductions on Outside mounts.

Length...
Measure the inside length of the window from sheetrock to sheetrock in three places, left, middle & bottom. Use the longest of the three for your Base measurement.

Next...

For INSIDE MOUNT...
There's no need to subtract or add. Use the Base measurement for the Length you will order.
The factory makes no deductions on the Length.

For OUTSIDE MOUNT...
Add 3 to 6 inches to the Base measurement to allow for mounting brackets and extra light coverage. This is the Length you will order.