Follow this guide to create and export your glTF files:
Export all your variations to zip files:
In the build section of your project, click on the IMport & Edit 3D model button.
This will create a blank product for you to work with and bring up the media library to select or upload your model. Add one of the zip files you created earlier by dragging it onto the window or finding it in the file browser.
After adding your first variation, you can then click on the 'Add A New Model Variation' button to add the rest of your variations in the same way.
You can add titles, thumbnails and edit all the properies of the variations in the right hand side property inspector panel.