This process is very similar to creating a page template or a module form. Start by finding Callouts in the Developer section. Here you will see a draggable list of existing callouts. Click Add Callout in the tool bar to begin. Callouts require a name and unique ID, and can contain any number of resources from any available form-field types (other than the callouts field type itself).
One unique attribute you will see on the callout form is the Default Display Label. You have the option to create a resource called "Label", and BigTree will use this as a label in the Callout view. You can also type static text into the Default Display Label box and BigTree will use that for a label when the resource label isn't available.
For each callout you create, BigTree will place a template file here: ~/templates/callouts/{callout-id}.php
Callout Groups
Callouts can be grouped in order to keep them accessible only where you want them. For instance, if you want to have some callouts available on the right side of your page and another set of callouts available on the left side of your page. In this case, you simply create a "Left Side" group and a "Right Side" group and assign each callout to the proper group. When you add the "Callouts" field type you can edit the field's settings and choose one of the groups that you want to restrict the user's input to.
Pre-BigTree 4.1
Prior to BigTree 4.1, callouts were restricted to templates and did not have grouping options. A template simply had the option to enable callouts or disable them. Bigtree 4.1 introduced Callouts as a field type that is available in modules, settings, and templates.