Gather evidence of the pothole as soon as you hit it. As long as it’s completely safe to do so, take photographs, measure the pothole’s width and depth and note anything else about it, such as its position on a blind corner, whether it was hidden from view, etc.
Gain as much information as you can on the pothole you hit as soon as possible after you hit it. As long as it is completely safe to do so, take photos using something such as a ruler to give an idea of scale, and measure the width and depth of a pothole. Keep it as accurate as possible – exaggerations won’t help.
If it’s not possible to do this for safety reasons, you may want to photograph the pothole from afar.
Make a sketch of the area, showing the location of the pothole as best as possible. This is particularly important if there is more than one pothole, as saying you hit “a pothole” without being able to identify the exact one won’t help you. Note also the lateral position of the pothole, ie was it in the wheel tracks, near the edge of the road, etc.
All of the above might sound daunting, but in reality should only take you 5-10 minutes.