1) Login to FTP using an FTP client like Filezilla.

2) Access your

WP installation/wp-content/themes/{theme_name}

3) Find and copy on local storage the style.css file from the theme folder.

4) Go back to themes folder and create a folder named {theme_name}-child (parent and child themes must be located in the same folder).

5) Open/Edit the style.css file from your local storage you just copied and add/edit the following lines:

Theme Name: {theme_name} Child
Template: {theme_name}
Text Domain: {theme_name}-child

These are required lines for child-theme declaration.

6) Move the style.css you've just edited to {theme_name}-child folder.

7) Go to

Dashboard -> Appearance -> Themes -> (activate) {theme_name} Child

8) Copy all your customized files to child theme (Do not forget to maintain folder structure)

By making changes in child theme you will overwrite the template files from parent themes and you won't have trouble (losing your customization) after theme updates.

Here is the most detailed explanation offered by Wordpress http://codex.wordpress.org/Child_Themes.