Start by creating a folder in themes/custom with a machine name for your theme, then add a themename.info.yml file that declares the theme’s name, type, and base theme.
Add a libraries YAML file for CSS and JavaScript, and start building Twig templates in a templates folder. Drupal falls back to parent theme templates when yours are missing, which lets you override only what you need.
Register your theme regions in the info file, add a logo and screenshot, and enable the theme through the admin interface. Most custom themes extend a base theme like Olivero, Stable9, or a starter kit to save time. Invest in a proper build process with a tool like Vite or Webpack to handle modern CSS and JavaScript workflows cleanly.