When a wood object is an abnormal shape, there are a lot of things that can be done to assist with processing the material. You can use simple modeling clay to secure the wood in place; many manufacturers offer accessories for this specific purpose as well including different cutting tables and rotary attachments. Jigs are another useful tool and can be crafted to accommodate each unique shape, making processing easier and more efficient with just a little creativity and imagination.
Overall, decoration of odd shapes requires skill and experience that is obtained over time, but there are a lot of online resources available to assist with mastering this.
-Josh Stephens, Trotec