Upload fk-google-tag-manager to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
Add a hook do_action('wp_body')
after opening tag <body>
<body>
<?php echo do_action('wp_body'); ?>
... other code
Go to Settings > General and set the ID from your Google Tag Manager account.
Use comma without space (,) to enter multiple IDs.