First, selecting everything on the spreadsheet and press ctrl+c to copy it. Then press the button highlighted below to enter the import screen:
Once you're looking at the import screen, press ctrl+v to paste in the information, followed by pressing the Start Import button:
Depending on how many items there are, the import process can take up to several minutes to complete. If there are any problems with the import, a popup will tell you what that problem is, and on which line of the spreadsheet you can find that item so you can make any corrections.