Step 1 Choose Select tool and paint the area you want to separate.
⬢In our case the helmet. (Note: Do not double click on the model. That would select the whole part)
Software used in this tutorial is Autodesk Meshmixer.
You can download Meshmixer at: meshmixer.com/download.html
⬢If you are using Fusion 360, you can download MeshMixer from there. In DESIGN workspace, visit the TOOLS tab, go to MAKE, 3D Print, select MeshMixer as a Print Utility and hit Download.