The easiest way to install the extension is to use Composer, just run the following commands:
$ composer require kodbruket/magento2-gtm
$ bin/magento module:enable Kodbruket_Gtm
$ bin/magento setup:upgrade
Enable the module under Stores / Configuration / Sales / Google API. Enter your container ID from Google Tag Manager and specify whether to use quote or order when tracking the transaction on the success page. When the module is set to track by quote it will try to set transactionID to the reserved order ID from the quote. If no order ID has been reserved it will fall back to the entity ID of the quote.
The module looks at the full action names catalog_product_index, catalog_category_index and checkout_checkout_index to decide which data to include in the data layer. It's fully possible to add or define other full action names directly from admin. Just set Use custom action names to Yes and define the action names you want it to look at.