Ok, that helps a little.
Suggest to get a large wash tub, big enough for the entire dog. A leash with a 3 point attachment, on dog and 2 solid points. You'll need a collar also
Begin with training as BluePaw has outlined.
Once you are able to get the dog into the large wash tub filled with warm water, attach the leash to the dog and 2 anchor points. The anchor point should be separated on opposite sides of the dog. The leash will allow you to give the bath and prevent the dog from leaving. Have plenty of treats handy to reward for each success. Look at tub, reward, step in tub, reward, attach leash, reward, get wet, reward, reward, reward, soap, reward..........and so on and so forth. Remember, the treat rewards do not need to be big, just enough for a smell and taste. Don't forget to give a some jackpot rewards for new achievements or big steps forward.