Below is a step-by-step guide to designing a simple skull using Tinkercad:

Create the Base:

• Log in to Tinkercad and create a new design.
• Drag a "Box" shape onto the workplane.
• Adjust the dimensions to create a flat base for the skull. You can make it roughly 50mm x 50mm.

Shape the Skull:

• Add another box for the main part of the skull. This box will be taller and wider than the base.
• Adjust the dimensions to make it wider than the base and taller to form the skull's shape. Aim for a rectangular shape with slightly rounded corners.

Form the Jaw:

• Add a smaller box beneath the main skull shape to represent the jaw.
• Position the jaw so that it aligns with the bottom of the main skull shape.

Merge and Adjust:

• Select all the shapes and use the "Align" tool to ensure they are properly aligned.
• Group the shapes together to merge them into a single object.
• Use the "Hole" shape to subtract eye sockets, nasal cavity, and any other details you'd like to add.

Add Details:

• You can add small cylinders or spheres for the eyes.
• Use additional boxes to create the nasal cavity, teeth, and any other details you want to include.

Final Touches:

• Once you're satisfied with the overall shape and details, group all the shapes together to form a single object.
• Adjust the size and position as needed to fit your preferences.

Export as STL:

• Once you're done designing, click on "Design" in the top right corner and select "Download for 3D Printing."
• Choose STL as the file format and save it to your computer.

This should give you a basic skull design in Tinkercad. Feel free to customize it further by adding more details or refining the shape to your liking!