How do I use a heater in Frostpunk?

Once you have researched heaters from the technology panel, you can click the temperature indicator at the top center of the screen to see the temperature overlay. Places that have an available heater can be switched on and off in the overlay by clicking the switch over the building. This is in addition to opening up the panel for each workplace where you may also toggle the heater.

It is worth noting that heaters are only for workplaces, not homes. Heaters only appear to be used when there are people working at the building.

You first need to build a workshop in order to be able to research such things. After you build a workshop, research Heaters that will consume 10 wood and will be researched in 7 hrs at the maximum rate (Time is according to the game time). After the research is complete, select a post and at the left side of the screen a panel will appear and on the panel, there'll be a thermometer symbol below the name of the post. Click that symbol and the thermometer symbol should turn from white to yellow and heater of that particular post will be enabled. The heater will increase the temperature of that post by 1 and will consume 1 coal per hour.