A messaging broadcast is a campaign sent via the Marketing Tools. It can be an Email sent to a group of supporters, an SMS, or a combination of both. It can also have conditional content or split tests.
Creating a new broadcast
To create a new broadcast, go to Marketing Tools > Messaging > Broadcasts.
This will list your existing broadcasts, if any. To create a new broadcast, you can either copy an existing broadcast or create a new one.
Copying an existing broadcast
This method copies across everything in an existing broadcast, including the message content and the audience query. This can therefore be a quick way to create something similar again. To copy an existing broadcast, click the copy icon 
next to the item in the list.
...
This will create a new broadcast in the list. From here you can edit the broadcast by clicking the 
edit icon. From here, check the various stages in the editor to edit your broadcast (Setup, Audience, Message, Testing, Sending, Reporting). For more information on these, click on the links.
Create a new broadcast from scratch
Or, to create a new broadcast from scratch, click the orange Create Broadcast button.
...