My first step was to look at a French Horn, front and back. I then figured out how all of the parts connect, and made paths in illustrator.
After bringing in my Illustrator file into Cinema 4D, I then used the splines and Sweep Nurbs to create the pipes in the French Horn.
This was my first pass at lighting and texturing.
WAY better. I lit the scene, and adjusted the two textures on the French horn to get great reflectivity.
With Sweep Nurbs, it is pretty easy to animate the start and end growth. I animated these, as well as the other parts of the French Horn.
I put collider tags on each of the pieces, and made a giant invisible cube with a collider body on it. I put the cube around the whole French Horn, and enabled collision!