Document everything. Write down the date, time and location of the animal and any other details that might be relevant. Take photos or videos to help paint the full picture.
Take the evidence to your local animal control agency or sheriff's office. For your own reference, take note of whom you speak to, so that you can follow up with the complaint in a few days.
Call the agency or the sheriff's office after you've issued your complaint for updates and to make sure they've followed up on the situation.
While it might be intimidating to contact the authorities, consider the alternative.
"Our most constant companions - dogs and cats - feel the effects of winter weather as much as we do, only they are often cast outside to weather the cold or a storm owing to a misconception that the fur on their backs will insulate them from suffering," the HSUS writes. "Without proper shelter, food and water, these domesticated animals' chances of survival in frigid temperatures is greatly decreased."
If you need specific advice, you can contact the HSUS here.