In order to import 'Sites', you will need to be an administrator and have access to the Account Settings. You can find an example file in the following article.

1. Go to 'Account settings' and select 'Import' and click 'Sites'.

OPTIONAL 2. Delete existing sites (if necessary)

If you want to replace sites already existing in the system with the newly imported ones, you can delete them prior to the import. If you have no existing sites or do not want to delete the ones already in the system, skip this step and go to step 3. 
To delete, collapse the 'field mappings' tab or scroll down to the bottom of the page to 'Upload and import'. Check the box for  'Delete all sites first'. Then click on 'confirm DELETE ALL SITES FIRST'

3. Define fieldnames

Before selecting the import file, define the 'Import file fieldnames' which needs to be aligned to your .csv file and click Save. Columns with a (*) are required in order to successfully import a file. Some fields have a '? icon' attached for further information regarding the setups. In order to obtain this information, hover over it with your mouse. 

Furthermore, you need to define the mapping types between single column, multiple columns and fixed value. You can find more information on how to use those different mapping types in this article

NoteBefore importing the .csv file, make sure that imported data are aligned with the information in the tool including Type...

4. Import the file

For the import itself choose the button  Choose File on the bottom of the page, select the CSV import file and click on open. After the upload the below-marked button becomes available, this confirms the import. 

The import should only need a few seconds, the green text in the following picture confirms a successful import.