Many retailers offer waterproof jackets, but in the instance that you'd like to waterproof a jacket yourself, the process is fairly easy. There are both sprays and wash-in treatments to help waterproof an article of clothing, although sprays are the easiest to use.
Clean and dry the jacket you want to waterproof. After it is dry, hang it on a hanger in a well-ventilated area.
Purchase fabric protecter waterproofing spray. These sprays can be found online or at most outdoor supply stores, such as REI.
Spray the waterproofing spray onto both sides of the jacket, following the instructions on the back of the can. Keep in mind that most of these sprays are flammable and make sure to avoid any open flames or small spaces.
Let the spray dry before wearing.
Pay special attention to areas not made from fabric, such as zippers and buttons. Stand as far back from the jacket as possible when spraying.
Waterproofing sprays are flammable.