First, create a Carrd Comments plugin on Common Ninja.
On the plugins catalog, find the Comments and enter the Editor.
When you're done creating the Comments plugin, click on the Save Changes button at the right top corner of the editor.
If you don't have an account on Common Ninja, you'll have to register first.
When you're ready to add the Comments plugin to your Carrd website, click the Add To Website button at the top bar of the Editor.
On the window that opens, click the Copy Icon to copy the plugin embed code. In the next steps, you will paste this code into your Carrd website.
Now, let's head to the Carrd website's editor.
On the Carrd editor, select where you wish to add the Comments.
Next, click the Add Item button (the “+” sign)
Choose the “Embed Element”.
Note: the embed option is available only to Carrd Premium users.
Now, simply paste the Comments embed code you've copied before in the Embed Code's properties panel, in the Code area.
Click on the Done button of the properties panel.
Click Save to publish your changes.
Please note, the Comments content will not show on the Carrd editor and will only be available on the published site.