I would start by adding the the new fields to the class.schema.php file.
Then add the new fields to the upgrade script in include.upgrade.php; but you could probably skip that.
Add support for the new fields in the class.entry.php file in the construct and add new getter/setters. Also, update the save / update methods.
Add new meta boxes or update the current ones in the class.form.php file so you can add /edit your new fields in the admin.
To display the new field data in the admin add it to the manage.php file.
To display the new fields in the template you would add new getters in the class.output.php file
The above assumes you’re adding new fields to the main entry table and not the addresses / phone and ect. tables. But the steps would be similar.
I think that is it; off the top of my head anyway. Hope that helps.