Ok here is a vid by pippin, its not a DR guide but you can see how he uses it. The basics on the DR is use friendly disguises when leaving spawn and keep DR out incase you get shot ect...it leaves friendly body on the ground. Next thing you have to do is know where the metal boxes are and when attacking stay around that area so you can use the DR a second time if needed. Once in enemy territory, ofcourse use enemy disguises and if you fail stab get your DR out fast and run. Heres the vid, ill keep searching for a better DR guide.
http://www.youtube.com/watch?v=i-pkwtU26UY