You can easily install Cookie-Script via Google Tag Manager (GTM) by following these step-by-step instructions.
It is assumed that you have already had experience with GTM and your GTM account is created, also you created a website container in GTM and added the GTM container snippet to your website as required: https://support.google.com/tagmanager/answer/6103696.
Implementing the Cookie-Script popup box in GTM
In your GTM Workspace create a new tag by clicking New tag > Custom HTML Tag. In the HTML field, insert Cookie-Script code copied from your item. Code should look similar to the one below:
<!--Start Cookie Script-->
</script> <!--End Cookie Script-->
Next, click Add trigger choose DOM Ready and apply a name to your tag.
Your final tag configuration window should look like this. (unique number of .js file have been changed ):
In the end, click "Publish" to apply your changes to your website.
That is all.