masteries: 21-0-9, do not skip feast, dangerous game, meditation, culinary mastery
runes: magic pen marks, ap quints, armor seals... (glyphs are flexible: ap, or mr depending on lane, armor quints + seals with ap glyphs are good in ad lanes, mr glyphs are standard versus ap lanes)
Start Q. Start Dorans ring usually. Most Cass max E, the fang, since this is her primary damage spell. It is a huge mana dump, you need to build +mana
Most Cass rush tear. It is common to go abyssal scepter after that in ap matchups; very good vs. ap threats. The magic pen helps you delete squishies quickly.
More great items: Zhonyas Hourglass, Rylias crystal scepter, deathcap, void staff...