Tinkercad

Tinkercad is a free and easy-to-use 3D design and 3D printing online app with tools to create designs from scratch or “tinker” with existing designs.

Pick a design you like in Tinkercad’s Gallery or visit your dashboard and click "Create new design" to either create a new design from scratch or import and modify an existing 3D model.
If you chose a design from the gallery, click “Copy and Tinker” to be redirected to the editor. If you only see “Duplicate and Tinker”, you will be redirected to your dashboard. In your dashboard, select the design and click “Tinker this”.
When you’ve finished modifying your design, click “Export” then “3D Print”.
Choose Treatstock and then “Continue to Treatstock”.
Customize materials and colors, compare prices and reviews, and select a manufacturer of your choice.