You'll get this down at the docks.
Kenny will look at a boat to see if it can be salvaged and tell you to use the near by telescope to check out the coastline. It is to the left. When you go to use it you realise it needs money to work. Go and find the yellow newspaper box which is a bit further to the right from the kiosk, and smash it with you tool. Then use the coin to use the telescope.
You will see someone (Molly) at the far end of the docks and tell Kenny to hide. They go in the kiosk and you approach them carefully, although you will be hit in the face and knocked to the ground. Clementine will come to your rescue and then Kenny will get involved, firing the gun by accident. This brings walkers and you all run into the alley.
Pass Clemtine and Kenny up to Molly and then move the dumpster at the back. Try to open the manhole cover and Molly will throw her pickaxe to you so you can get it open.
You get the achievement when you drop down into the sewer.