You can add custom pages via Pages&Menus within the administration panel. Added pages are displayed within your site's general design/layout ( footer, header, sidebard and etc.)
In case you would like to have fully customized page with 100% own custom content ( your own footer, header, sidebar, background and etc.), you can use external custom pages.
Requirements: text editor, FTP client or access to control panel.
RewriteCond %{REQUEST_URI} !/custom_page\.html
Make sure to replace custom_page with the name of your own custom page. This rewrite condition tells the system not to block your new page. If you don't add this condition, you will get “Page Not Found” error if you try to access it.
Save the changes done in .htaccess file. Go to your homepage and try opening your new page.