To be able to complete this process, you must have the Google Tag Manager configured on your site, click on this article to configure in case you didn't.
You also need to have the Facebook Pixel script in hands to move forward.
If you already configured the Tag Manager and has the Pixel script, follow the steps:
1. To login on your Google Tag Manager panel, click here
2. While logged in on Tag Manager, select the option "Add a new tag"
3. Choose a name for the tag and click on "Tag configuration"
4. Search for "Custom HTML" and click on it:
5. Paste the code and than click on "Triggering":
6. Select the option "All pages":
7. Click on "Save":
8. Click on "Submit":
9. Click on "Publish":
Your tag must be already added on your site, I suggest to download the Google Chrome extension called "Facebook Pixel Helper", it will show you if the pixel it's working on your site, check an example below:
If you still cannot see the pixel on your site, try to clear your browser cache.