01) Access the Control Panel
02) After login click on Catalog >> Select Category / Sub-Category >> Select the product and click on the EDIT / NEW PRODUCT button
When registering or editing a product you have the field Products by Categories select the categories and subcategories that you want to relate to the product.
03) After clicking the Preview button.
04) Click on the Update or Insert button.
Videos:
https://www.youtube.com/watch?v=1FC7I9SKqeo&feature=youtu.be
https://www.youtube.com/watch?v=58sK6VUBPaM&feature=youtu.be