Use the WYSIWYG editor to enter the content you would like to be displayed in the module. In place you would like to use the content of the custom field you use the shortcode for the field. The shortcode is {field ID}. The ID is the id (number) of the custom field and can be found in the Administration Menu -> Content -> Fields -> Last column of the fields table.

To enter the shortcode into the module content you can either do that by writing the shortcode or by using the button and modal window with a list of all custom fields.

Example: {field 11}

If there is no field with the given id, the shortcode will be displayed unchanged!