Start up your computer,
Open the Calibre program.
Connect the Kobo to the computer with its USB lead.
Select "Manage" (or "Connect") if the Kobo asks you what you want to do.
The Calibre program should detect the Kobo and you will see a "Device" icon next to "Library" in the top row.
There will also be an "On device" column to the left of the book titles.
To add a book onto the Kobo, right click the book's title, hover over "Send to device" and left click "Send to main memory".
The book will then be copied over.
When finished, close Calibre and disconnect the Kobo.
Kobo will take a moment to update itself.