I tried these other suggestions and they didn't work well enough (it was a really bad case, we had a horrific winter). Eventually I had to call my vet and she gave me a pink ointment that cleared it up SUPER well.
Shower your horse only with water and an exfoliating glove. You scrub the dead hairs and the mangy places. It take quite a lot of time so you have to be patient!
In addition to water, you can use a very soft and disinfecting formula to scrub. I was used to do this and really happy with the result :)
Um touchy subject... It is essential to disinfect the "wounds" everyday until the mud fever is gone. Do not let the wounds wet or moist so dry them with a clean towel.
Vaseline is effective, you can use it twice a day on each wound.
And let your horse in a dry zone. If you cannot put your horse in a box, put wood boards on the muddy or wet zones of his field. It takes some time but you won't get out of it if you let your horse in moist grass or mud!
Be sure your horse stay in a dry place. Then clean the areas affected by the mud fever and dry them. After you can apply a greasy cream (vaseline for example). Do it as often as necessary.
To me it is better to protect the wounds after you put cream with standing wrap.