UPDATE: I didn't like the odd shapes of the raindrops in my first attempt, directly below. So I changed the way I created the random shapes, by using a simple hard round brush to create a defined brush set, then turned on the shape dynamics, scattering and smoothing brush presets, to create the rounder shapes. I also filled the window pane with tiny droplets as well as the scattering of larger droplets. This makes the pane look wetter and more realistic.
Here is the update:
My first attempt is below. The tutorial I used can be found at this link.
I used "distort/glass" to make the scene slightly blurry, as if the pane was wet from the rain, then used noise and other Photoshop tools to create the raindrops. I used a brush to create the falling rain. The result is acceptable, but I want to learn some different techniques to create this rain effect. My Photoshop is directly below, and the original photo is below that. [See update above, using a better technique].
Here's the original photo before the Photoshop applications.