Install WordPress locally using tools like XAMPP, MAMP, or Local by Flywheel. This allows you to test your theme without affecting a live site.
Create a new folder in the wp-content/themes directory. Inside, create the style.css and index.php files as the core of your theme.
Include files like header.php, footer.php, and functions.php. These files help structure your theme and add functionality through WordPress hooks.
Write custom CSS in style.css to design your theme. Use responsive design to ensure it looks great on all devices.
Activate your theme in WordPress and thoroughly test functionality. Use tools like Theme Check and Query Monitor for debugging.