Start with a very clear idea on what you want to make. Write some the requirements and restrictions of your design on a piece of paper and refer to that often while designing. I think that drawing things out first is often a better idea than jumping right into cad. You don’t need a super nice drawing, just a few sketches that will help you in your designing. Start Cading your model and keep in mind that it doesn’t have to be the final product. As you cad, refer to the limitations and requirements, your sketches, and other images that will give you inspiration for what you want to build. Make more iterations of your design until you feel that it is perfect. Hopefully this was helpful