Custom Fields Installation?

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I’m am looking to add the the custom fields to my installation, and I found this code in the “tips” section.
Two Questions:
1) Can you clarify where this code needs to be added?
2) It appears this is several years (and versions) old.  Is this still correct with the latest version?

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You have to add this code to you theme’s functions.php file. Please note this require at least moderate knowledge about PHP language.
Yes, that snippet are still relevant.

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