Ok, we need a little more information and clarification.
1. Which Nexus 7 do you have? The Nexus 7 with 3G?
2. Who or what is saying "Disable cell standby"? Is this an error message that the tablet is displaying?
3. How long does your battery last on a single charge? It's important to know what kind of battery life to expect--maybe what you think is a fast drain is actually normal. In general, the Nexus 7 should last anywhere from 6 to 8 hours of total screen-on time with one charge, depending on what else you're doing. If the screen isn't on, it should last much longer (I can go several days without charging if I don't turn the screen on).
4. To turn on Airplane Mode, swipe down from the TOP RIGHT of the screen to get to the Quick Settings, then tap Airplane Mode. This immediately turns off all radios (3G, Wifi, and Bluetooth). Once you are in Airplane Mode, you can still turn Wifi and Bluetooth back on, but 3G should remain off. As mr_nobody mentioned above, you're not going to notice an immediate difference in battery consumption--try waiting for a day on Airplane Mode to see if the battery life is better.