There are two ways:
1) Getting a substance called Putty, smear it on the location you want and then paint it the mud color (brown, black, etc.)
2) Get a piece of chalk and scrape off the dust color of it ( I prefer a red chalk). Once you do that, put one drop of water on the dust and mix it together with a brush. After that, smear the substance with a brush on the location you want. In a couple of minutes it would look most realistic and guarantted.
NOTE: USE A LOT OF DUST FROM THE CHALK, THE MORE YOU USE THE MORE DENSE THE MUD WILL LOOK!
Here is how it should look like on the Sherman I built a couple of weeks ago, ENJOY!
-ERAD
