First, it's best to find out the difference between Projects, Mailings, Versions and Templates.
Select the project you would like to add your mailing to from the projects page. Hint: click on the campaign name to open the campaign.
Use the ‘add a new mailing’ button on the top right of the screen to add a mailing. Hint: Alternatively you can clone an existing mailing by clicking on ‘actions’ then ‘clone’.
Fill in the details of the mailing and select a template to use for this mailing.
You can go back into the mailing and change the template at any time and you can use the same template across multiple projects and mailings. You can have multiple mailings per project.
Every mailing must have at least one version. Define your version/s by giving it a name and description. You can have multiple versions per mailing.
Once you are done filling in the above fields click ‘Create Mailing’.
You will now be able to see your new mailing listed along with the other mailings in your campaign.