Complex sentence: During 2005 Sinitta then helped Cowell one more this time choosing the final three groups, in 2006 Sinitta helped Cowell select Leona Lewis this becoming the second winner chosen by Cowell and Sinitta with Lewis becoming one of the most successful acts of the show.
Split into simpler sentences: During 2005 Sinitta helped Cowell one more, this time choosing the final three groups. In 2006 she helped Cowell select Leona Lewis, who became the second winner chosen by them, with Lewis becoming one of the most successful acts of the show.