I've been working with Google analytics since now, and as a part of the new update, I need to implement everything using Google Tag Manager. How do I use my custom dimensions and metrics of Ga in GTM? How's Data layer connected with GA's dimensions and Metrics? I'm very new to this tools, so please pardon me for any mistakes in the question and feel free to edit it. I'm confused with the other sources in Google and other documentations regarding these topics.
Create a Universal Analytics tag, click the "enable overriding settings in this tag" checkbox, go to more settings, custom dimensions, click the "add custom dimension" button, enter in the left field the numeric index as obtained via the GA property settings and in the right field your value/variable. Similar for metrics.