First of all, you need a .xls, .ooc or .csv file which contains the data you wish to upload. Each row of data will be saved as one article/k2 item/web link/contact/admintools redirect in Joomla!.
Once you have installed the component you need to set certain parameters before you can upload any data. When uploading articles these parameters are called "First Data Row", "Intro Text Format" and "Content Text Format".

"First Data Row" refers to the first row of data that should be saved as an article. If you have column descriptors in row 1 you should set this value to row 2. "Intro Text Format" and "Content Format" are parameters that allow you to specify how the data will be presented in the articles. At least one of these parameters must be set in order to upload articles and they should contain references to the columns from the Excel workbook in placeholders like so:  {A}

Download the demo spreadsheets to use with the pre-installed configurations from here.

Here is a short video to demonstrate how easy it is to upload Joomla core articles: 

Content Uploader Pro - Yachts Demo