This achievement is story-related and cannot be missed.
*** Spoiler - click to reveal ***After attaining the "Sneaky" achievement, leave the cabin. You will automatically return to the shed. Get ready for some major grimacing! Pour hydrogen peroxide on Clementine's arm (oww) and use the needle to start stitching the wound (owwwwwww). Repeat with the needle three more times (owwwwwwwwwwwwwww). During this process, you may be thinking "I probably should have boarded up the corner of the shed where I broke out." Before you can even bandage the wound, though, a walker crawls in and grabs you. Fight off the zombie using the quick-action prompts - keep an eye on the ground for objects you can throw or hit the walker with. When you prevail, the noise will have attracted the cabin's residents, who come to meet you in the shed. "Still. Not. Bitten." will unlock at this point, and is actually one of the verbal responses you can choose when the residents confront you.