If your unsure of the horses personalitys, or how they will react to bathing/water/hoses, etc. I would start with a soft brush on the face, with them in the stall and you on the outside for safety. Just work up from there :)
A metal shed brush, if used gently, can help remove a lot of surface dirt. And curry curry curry.
Then, once you get to know them a bit better, I would start with bucket washing and work up to spraying water and hoses. It's really not necessary if you know they do well with water and hoses already.
If there manes and tails are really knotty, wash them first and let them dry. I personally use Cowboy Magic when it comes to detangling nasty knots. Work slowly and don't use scissors unless they got a horrid rat-nest thats impossible to untangle.
My gelding Loki had one. It was a knotted hairball, small though and it didn't affect his mane at the least when I cut it out.
Be patience and it'll go faster than you think :) Use a gentle shampoo if you can as there skin might be dried out from all the dirt and mud.
Flax seed oil and fish oil is supposed to be really good for the coat (I asked in another thread ^^) when given in small quantities.
Oh! And paprika is also supposed to be quite good.
Wait! I'll fix it....