What I use is mildshake straws (bubble straws) between each layer and drive a strong wood dowel through the middle. Never had any problems even with a 5-tier cake.
I'm not sure what kind of guitar you are wanting but I use any cake or icing. I bake a sheet cake and carve it in the shape of the guitar and I use rct for the neck of the guitar, kids love that. I...
I know what you mean about getting freaked out when it comes to making car cakes. 1st thing I would do is google tutorals on how to make a car cake or youtube has a lot of tutorials. You can buy the race car pan and carve...