Use Q when enemies are last-hitting, E to pierce through minions, and W to deny enemy engages/escapes.
Opt for a RoA + Riftmaker build over full tank to strengthen your laning and late-game R stacks, and Axiom Arc for ultimate uptime.
Be aware that your R stacks amplify Sett's ultimate damage, and remember that Cho'Gath's laning phase is stronger than often perceived.
Stats
Q
W
E
R