Basically once the child theme is activated, WordPress will look for the template files in the child theme folder first. If the file doesn't exist in the child theme, it will fallback to the parent theme folder. In other words, if there is a "header.php" in the child theme, WordPress will use that "header.php", else it will use the "header.php" file in the parent theme.
Step 1: Create a new child theme folder
To start a child theme: on your computer desktop, create a new folder (eg. "themify-ultra-child")
Step 2: Create the child theme style.css
In the child theme folder, create a new CSS file in the child theme folder and name it style.css.
Paste the sample code below in the style.css file and change the theme name and template name as instructed below.
Theme Name: Themify Ultra Child
Description: Child theme for Themify Ultra
/* write custom css below */
Theme Name (required) = use the parent theme name + child to make it easy to identify (eg. "Themify Ultra Child")
Description (optional)= you may enter any text here
Author (optional) = your name
Template (required) = name of the parent theme folder (in this case, it is "themify-ultra")
Then you may add any additional custom CSS as you want in the child theme style.css (optional).
Final Step) Installing/activating The child theme
Once you have the child theme done, zip it (eg. it should be compressed as "themify-ultra-child.zip")
Login to your WP Admin, then go to Appearance > Theme, install the child theme zip (the same way as installing a WordPress theme), and then activate it.
NOTE: Make sure the parent theme is installed (ie. if the parent theme is "Themify Ultra", make sure it is installed in your WordPress site).
Advanced/Optional: Overriding Parent Theme Template Files
Overriding template files is intended for developers who have PHP and WordPress coding knowledge. It is only required if you need to override the parent theme template files. If you are not sure how to do this, consult a developer.
To override the template files in the parent theme, open the original template file and save a copy to the child theme folder with same file name and folder structure. Basically, the file structure has to match with the parent theme.
let's say you want to edit the loop.php file in the parent theme and it is stored in the "includes" folder
open the loop.php file and save a copy in the "includes" folder of the child theme
edit and save the loop.php in the child theme.