On left hand menu, search "Data Import Wizard" or go to Administer -> Data Management -> Data Import Wizard
Click the "Launch Wizard!" Button
Under "What kind of data are you importing?", Click "Custom Objects"
Scroll to and Click "Products"
Click "Add new records"
Drag & Drop CSV file or Click CSV button
Map any relevant unmapped fields
If everything looks like the image below, click Start Import
When done (Progress 100%), you will see records processed.