You'll gain this achievement with story progression, so don't worry about missing it!
Once you've entered Goulston Street, Holmes will tell Watson to get something to put the fire out, from there you'll take control of Watson and need to go and find a few items,
The items are;
• A pole
• A hook
• A rope
Once you've collected this items you need to combine them in your inventory, once you've done that you'll need to find some water, a watering can is up on a ledge above you, climb a box close by and use your newly combined item to retrieve it, once you have it run back up the street and fill it with water and then return to Holmes.
If you find yourself stuck on a particular part of the game be sure to check out this excellent video walkthrough of the entire game, the users youtube channel can be found here;
http://bit.ly/ih2LIC. Alternatively I've linked in the first video below and you can use that to reach his channel.
* Credit to adventuregamefan for the video