I know this is late, but some more details might be helpful. Are we talking about an electric/solidbody guitar, or an acoustic? I don’t know about acoustic guitars, cuz they seem like their own thing, but with a solid body guitar, I imagine a wide chisel to get the walls straight, then sanding by hand with increasing grits of sandpaper to just clean up the surface finish. One of those sanding sponges might be helpful.

Upvoted because it’s not a woodworking community without “my first dovetail” posts. Nice job!

I know it’s not fine furniture like most of the other posts here, but I was pretty proud of myself for being able to throw this together in an afternoon with only stuff I had laying around.