How do you add C3 Tags inside of Google Tag Manager?

Adding C3 Metrics via Google Tag Manager is a simple process.

Site Tag

To add the Site Tag, select 'Custom HTML Tag' and paste your tag into the HTML section.  Leave 'Support Document.Write unchecked.  Add your Firing Rules to have the C3 Site Tag appear on every page and a Blocking Rule to have it not appear on the Order / Thank You Page.

Conversion Tag

To add the Conversion Tag, select 'Custom HTML Tag' and paste your tag into the HTML section, leave 'Support Document.Write unchecked.  Add your Firing Rules to have the C3 Site Tag appear only on the Order / Thank You Page.

For a Conversion where there is no data being passed to C3 (your tag only has a c3_type parameter), once the tag is added Publish the changes and you are all set!!

For a Conversion requiring data to be passed to C3 (your tag has a c3_transaction, order_amount or account_id) parameters, you will need to utilize Macros.

Within the Macros tab look for the different type of Macros and locate the ones you need.  

You can easily set up new Macros and there is TONS of instruction available from Google, such as  this for E-commerce tracking.

Macros are entered within two curly brackets, such as follows {{macro}}

See the example below: