You need to customize one of your huts. For example: I build a pink hut and then I can tap on the hut and will find a button which says "customize". You will find a selection of different hut customizations and will be able choose a chimney for example!!!Another way would be to directly search in your build menu (the button in the right lower corner with the hammer on it) and choose the Smurfs and then Customizations section
You do this by tapping on one of your smurfs' houses. There you should see a button offering to customise it. Click on that and you will see a list of blueprints. Some of them you already own and can use, others first have to be won from Lucky/message in a bottle/etc or bought. Pick a blueprint of a chimney you like. Then you should get the option to choose which house you want to add the chimney to. Choose one and click on it. It will then build the chimney for however many resources it costs in whichever time it takes. (For example 80 stone, 2h. You will find those infos in the blueprint description.) You will need a smurf for that, so if all are busy harvesting, you will have to wait for them to finish or cancel one crop. Before you do this, it won't even let you pick a customisation.
Hope that helps. If you have any questions, feel free to ask
Edit: Twinkiepie was a little faster than me, oops!