To get you started, here’s a child theme template that includes a functions.php file. Download and unzip this template, then change the style.css file as per the instructions in your text editor (you’ll probably have to right-click and select “Open with” to use the right program). You’ll need to change the template name and import URL to reflect your parent theme’s information.
Download Sample Child Theme
Make sure that future-you appreciates the past-you – don’t make site updates complicated, and don’t tempt future-you to avoid doing them. Add custom code to WordPress in a way that’s safe, moves from theme-to-theme, and makes it easy to debug if something ever goes wrong.