1. Go to Products > All Products.
2. Click on "Edit".
3. Click on "Variations" under product data.
4. Select your color and size or any other variation available.
5. Click on "arrow down" to expand the section.
6. Tick on "Manage Stock" and insert the available stock quantity.
7. Click on "Go" beside add variation field.
8. Choose any other color and size or any other variation.
9. Insert SKU (it need to be unique, you may insert something like 5-61805-2 or any other relevant code), price, tick on manage stock and insert quantity available.
Repeat step (step 7 - step 9) for any other variations available.
10. You can click on "Remove" to delete it.
11. Click "Preview Changes" to check your update. If everything is okay, click "Update" to publish it to the front end.