I feel the best way to turn cena heel is to put him back in the title picture. Everyone will be like "oh here we go again".
Have him win the strap at a PPV but let him cheat to do so, have zack ryder come down to question him on why he cheated
and then have cena destroy ryder.
Make the match a no DQ and have cena take a chair viciously to his opponent to the point where the ring fills with officals trying to stop him.
Zack Ryder will come down and try to calm him down and bam a chair shot for him too.
Also since punk is super over right now it would be great if he was the one that cena took the title from.