Importing data to custom attributes is very simple - the column of the field has to be named according to following pattern:
underscore symbol + attribute's alias
For example if the alias of your custom attribute is "additional_features" then the name of the column in CSV file should be "_additional_features".
In addition, if you are going to import data to fields with predefined values (i.e. select, radio, checkbox), then as a value you should provide the exact name of the value or list of values separated with a comma.
For example, if the checkbox field has available options [Alfa|Bravo|Charlie|Delta] and your product has values "Bravo" and "Charlie", then the content of appropriate cell in CSV file should be: "Bravo,Charile".