A default package is a package that is automatically included with any reservation which uses the rate that it is attached to. It is set up under rate plans.
First, create the package following the instructions in this article or this video
To attach the package to a rate, go into 'Configuration', and then 'Rate Plans'
Select the rate plan that you would like to edit, then within that rate plan, click on 'Edit [rate name] Settings'
Click on the dropdown menu next to 'Default Package' and select the package you wish to add. Then click on 'Save' in the top right corner.
Repeat these steps on any other rates or rate plans that you wish to attach the package to.