She always needS to be reminded when it comes to preparation because she is not an organised person at all / not organised at all. S on the end of present simple verbs for he/she/it. Because should not begin a new sentence here. Self is unnecessary. Person is also unnecessary, but isn't unnatural.

as Claire has written ; 'warned' is maybe used if there are serious consequences OR '. . not at all organised'

"Because ... at all" is a subordinate clause, not a sentence. "needs" Other than that, the only change I would make would be to say "self-organizing" rather than "self organized".