Besides the basics, bucket soap etc. It really is easier if you can get someone to help you. One of you hold the horse the other give the bath.
If the horse has never been bathed before, start off by getting them used to the running water from the hose. Once he/she sees what it is and isn't freaked out by it. Start running water on the front feet working your way up the leg. DO NOT just start getting the horse wet. You want his first experience to be half way pleasant so go slow. Wet up the front leg on one side to the neck then down to the back. Go down to the back legs then up to the rump. Be careful around the tail area and dock. Some horses (mares in particular) are very sensitive and will sometimes collapse the back legs if too much pressure is sprayed around and under the tail.
Soap him up good including the tail/mane with a sponge. I use a rubber curry or jelly to work the soap in if they are very dirty. Rinse and repeat on the other side.
I use a sweat scrapper to hurry the drying process. I also don't let them free till they are completely dry. Mine will go roll in the nearest dust pile if they aren't dry. Then you have to do it all over again
ETA: I add the soap to the water in the bucket. If you just rub soap on them your really wasting the product and getting to much soap on them. Think car wash
Also, when you first start with the hose. Keep the water running on the front leg till they relax a bit before you start up the leg to the chest and neck. Have fun and don't get too wet