This guide may yield different results depending on the game, skill level of the summoner, and team composition. This guide is not intended for ranked games, 3v3s, or co-op.
Usually when I play this build I get a lot of kills and have enough defensive gear to tank or off-tank. The keys to making this build work are: focusing on keeping feast stacks at all times, using Q and E abilities, and focusing squishies.
Because Cho can get tons of assists and/or kills, for a purely tank build I would normally buy Leviathan to start out with; Mejai's Soulstealer for purely AP. However, getting a Catalyst the Protector early game helps with all early game mana problems and gives a little bit of extra cushion until you reach level 6 and can stack feast.
Boots and defensive items depend on the team you are fighting and your play style. If you aren't having problems with AD characters on their team skip the Thornmail and get Banshee's veil or Aegis of Legion. If you feel that Cho runs too slowly get Boots of Mobility. If you are heavily relying on Cho's auto-attack then buy attack speed boots.
Rod of Ages and the Deathcap are very important in this build and should be bought nearly every time. They give you enough AP to do plenty of damage to the other team without wasting a ton of gold. If necessary you can replace the Deathcap with Zhonya's Hourglass.
Guardian's Angel is one of my favorite times for Cho because it makes him nearly invisible and warns the team not to f*ck with you. It can be replaced with more AP-heavy items if the game is going smoothly and you're not getting killed very often.
Red- Flat magic pen
Blue- Flat mana regain
Yellow- Flat ability cooldowns
Runes and masteries should be set up in a way that helps your play style. This character will have so much health by the end of the game just from his Feast stacks that I don't recommend using too many defensive runes or masteries. Remember, runes and masteries mostly only help you early game, so don't worry too much about your set up. Try out different things until you find what works for you best.
The way I build:
Because you will have a lot of AP, it is good to choose masteries and runes that help with early game mana problems (so you can spam Q). I prefer the typical mage set up for my masteries.
I do magic pen in red and quints so I don't have to buy many MP items. This starts me out with 14 MP. Because Cho can lane for a long time because of his health, I do mana regain in blue. This prevents you from having to go back so much for mana in the beginning and it stacks with your masteries. In yellow you can do either ability power per level or CDR. I like CDR because you can get feast stacks up faster after you die, and it use it more often on champions.
Start off with your Q ability so you can harass often. Q can also help you get kills when a champion with low health is running away.
After that prioritize E. Then focus on Q. Lately game, focus on W.
Of course, always learn your ulti when available.
Your W ability doesn't do a lot of damage, so try not to rely on it for anything other than silencing.
The E ability should automatically toggle on, but always double check!