Turtle and spotted salamander?

How do I preserve a painted turtle shell? I found a dead one and left it under a cup with a rock on top. There was an ant hill and they are slowly cleaning its insides. However, the disappointing thing is that the underside of the shell is losing color and looking like its shriveling up. What's going on? Why is it losing color? And how do I stop it and if possible reverse it? My next question is how do you assemble a skeleton? I found a Spotte salamander corpse an want to assemble the skeleton for display. How do I do this? Thanks. Sorry if these are stupid use toons but unwanted my answers from a knowledgeable community of taxidermists as opposed to google.