First things first, always check the care label on your coat. Every piece of clothing is made differently and requires different procedures. If your coat is machine washable, follow the next steps:
Remove excess dirt and grime from the raincoat, then pretreat stains if necessary. Also, it’s best to clean the washing machine detergent dispenser to remove old residue before tossing your coat in there.
Zip up the raincoat, fasten all buttons, close any velcro straps, and make sure all items are removed from the pockets before putting in the washer.
Add a free and clear (or otherwise gentle and safe) detergent to the washer. DO NOT use fabric softener as it can damage the waterproof treatment on your jacket.
Run the washer in cold water on a delicate or hand wash cycle. You may want to add an extra rinse cycle to make sure all detergent is removed from the coat.
If you wanted to re-waterproof the coat, this is where you’d do it. If your coat doesn’t have a fabric lining, you can put it back in the washer with a durable water repellent. Run that in cold water on a delicate cycle. If your jacket does have a fabric lining, use a spray bottle to spray the durable water repellent on the outside of the coat. While your gear is still wet, spray on the waterproofing, holding the bottle about 10 inches away from the clothing to get an even coat. Then move on to drying.
It’s usually best to air dry your raincoat. Put it on a hanger or lay it flat in a well-ventilated area. Depending on the raincoat, it could be okay on a low heat setting in the dryer, just pay attention to the care instructions and make sure it’s a no heat or low heat setting. If you decide to re-waterproof your coat, air drying (specifically hang drying) is highly recommended.