I've managed to generate new fields but I can't add them into the report templates. How can I do that?
Make sure they are marked as searchable. On your: civicrm/admin/custom/group/field?reset=1&action=browse&gid=1
(assuming it's your first Custom Data set)
Check the Searchable? Column; If you toggle that to Yes - then reports that pull in custom fields will show yours at the bottom of Columns and Filters - e.g.: