The best and easiest way to implement Google Tag Manager in WordPress is by adding it directly to the header file of your theme or you use a light weight plugin that inject codes to the header and body.
Here’s the Step-by-Step Guide for WordPress
1. Make sure you follow this guide first. How to Create Google Tag Manager Account
2.Next, copy the script for the <head>, and login to your WordPress Dashboard go to Appearance > Editor then press CTRL + F (for Windows) / Command + F (for Mac) users and find “header.php”
3. Usually, majority of all WordPress theme has header.php exposed on Theme Files. If it’s available, click the header.php to open it.
11. Paste the first code right before the closing tag </head>
12. Then, paste the second code right after the <body> tag.
4. Save and your done. 🙂
To verify if Tag Manager is working. Just follow this guide: How to Verify Google Tag Manager Installation