https://www.youtube.com/watch?v=M33CszMLPSg&ab_channel=Sketchbox
You can also download the model files from Poly, and add them to your Sketchbox Mesh Folder: %USERPROFILE%/Documents/Sketchbox/Import/Mesh or C:\Users\<your-user-name>\Documents\Sketchbox\Mesh.
Note: As of 2018.12.10 some TiltBrush files can no longer be downloaded from Poly. To export a TiltBrush file for use in Sketchbox please follow the instructions below:
.fbx is a popular format for 3D modeling, and can be used in applications like Blender and Maya.