MeshLab is an open source system for processing and editing 3D triangular meshes. It provides a set of tools for editing, cleaning, healing, inspecting, rendering, texturing and converting meshes. It offers features for processing raw data produced by 3D digitization tools/devices and for preparing models for 3D printing.
Uploading
When your mesh is ready to upload, go to Filters → Smoothing, Fairing and Deformation → Export to Sketchfab. A pop-up will appear for you. You'll want to enter your API Token in the Sketch Fab Code field, as well as a title, description, and appropriate tags.
You can also mark this export as Private (if you have Pro membership on Sketchfab) or Publish if you want to skip Draft Mode and share your model with the Sketchfab community.
In most cases, we would recommend you keep Auto Rotate enabled.
Then, click Apply and your model will be uploaded to your account.
Decimating a Model
This is especially relevant for high-resolution 3D scans/maps. How to reduce the polygon/face count of a model for better performance on Sketchfab:
Open your model in MeshLab. For example, drag and drop the OBJ file from your 3D scan into the window or go to File → Import Mesh... and navigate to your file.
Go to Filters → Remeshing, Simplification and Reconstruction → Simplification: Quadric Edge Collapse Decimation (with texture) (if you're using a texture).
Choose a Target number of faces. Try 500,000, or even lower if the model quality doesn't deteriorate too much. Then click Apply.
When it finishes, Export mesh as... (e.g. OBJ) and make sure the TexCoord option is enabled if you're using a texture.
Then, you can upload the new mesh to Sketchfab.