Thanks for comment Paul! I talk about it in this lesson with the Wes tune Road Song.
What I can say about the muddy sound is that I like it a bit muudy and that´s why I play with the thumb but one have to have a clear sound from the amp and the guitar so it don´t be to muddy and also play with a flick in the thumb and feel how hard you should play to get the best sound.
If you mean hitting other open strings then is my advice to really slow down and look where and with which fingers you must mute to avoid the open strings to ring. That can be quite tricky with some tones but when coming through the issues it will come natural to you.
...and as always have patience!