Measure and record these 3 dimensions in that order. Width is horizontal (across the window), height is vertical (up and down), and depth will be a few inches straight back into the window pane.
Consider how much privacy and light control you want in order to get the right outside mount coverage and enough overlapping on the sides. You want to maintain your privacy and cover the whole window fully. The more coverage, the less light seepage from gaps on the sides.
The hardware height must be added to the height you choose, so the blind will not be too short when installed. Also, if you choose a valance, you won't want it to be too low where it will hang down into the open window.
You will mount on the drywall above the window opening or above any existing trim. Take into account obstructions around the window such as trim depth, AC units, and window cranks.
You can mount your blinds to the trim as long as it is flat and meets the minimum requirements for an outside mount.
Mounting on the trim of the window is the easiest route because all you need to collect is the size of the trim dimensions. Also, trim mounted blinds fit the architectural lines of the window and look custom.
We have helpful measuring guides for any style of window treatmentmeasuring guides