The best way to customize a WordPress site is to use a child theme.  That way the customizations are not lost when the parent theme is updated. Additionally, if you make a major mistake changing the files and the site goes down, you can just delete the child theme folder and the site will come right back up using the parent theme.

Here are the steps I used to create a child theme for my site which uses the WordPress Twenty Fourteen theme.

1.  On your local computer, create a new child theme folder (example:  c:\website\childoftwentyfourteen).
2. Add a style.css file to the new child theme folder as follows:
   • Open Notepad++ and save a blank file as style.css—selecting file type of Cascade Style Sheets File (.css) from the dropdown list.
   • Add core code information to the very top of the style sheet file as shown below.  Note:  The template value is the folder name of the parent theme folder.  All other values can be anything you choose.
   • To import the parent theme styles, add a first line of code as shown below (change the name of the parent theme as needed).
3. Using FTP, upload the new child theme folder with style sheet to the themes folder of the WordPress installation (WordPressFolder\wp-content\themes).
4. In the WordPress dashboard, go to Manage Themes and activate the new child theme.

You are now ready to customize your site!