This is the guitar cake that I made for my friend's graduation party after much thought and research (including this site!).
For the guitar handle I used some spare wood that my dad had and cut for me. I painted it with a black gloss paint and glued painted silver noodles onto it for the frets. For all the rest of the pieces on the handle I used wooden craft things from Michaels and painted them to look like the things on a real guitar. (I painted the tuners white.)
The strings on the guitar were clear fishing line hot glued at the top and bottom of the handle to keep them taut. The silver rectangle things are connected to the handle by the strings and laid onto the finished cake.
For the cake I used two boxes her favorite cake mix Funfetti, and made two 13 by 9's. After they had cooled completely I froze them so that it would be easier for me to shape them. When they were ready I shaped them by using a template I had made, cutting the layers one at a time. After they were cut a I used a homemade butter cream frosting for the filling between the layers and for the crumb coat which i strongly recommend doing! A couple of hours later, with the cake nice and chilled, i frosted it. Frosting the cake was pretty easy since I wasn't painting or airbrushing it but the only difficulty i had was getting it to look smooth. After lots of trips to and from the refrigerator (trying to keep the frosting from melting) I found that a butter knife wet with warm water works to smooth out all the bumps.
When it was all smooth I used a piping tip to outline the different parts of the guitar and make it look cleaner. The chocolate pieces on the guitar were rolos. I cut two rolos candies in half and used them for the top of the guitar and i used one whole rolo candy for the bottom of the guitar; secured with a toothpick.
I assembled the cake at her party by placing the guitar handle on some wooden stands and securing the silver rectangles onto the cake with extra frosting. I also piped along the bottom of the cake to give it a finished look.