This can currently only be accomplished by throwing Freeze Bombs at Tar. Before doing so, it is advised to create Tar-cataracts and/or deep lakes of Tar at first so you won't waste too many Freeze Bombs and perhaps get a maximum of Coal Nodes.
Since one Freeze Bomb can freeze up to 7x7 portions/"blocks" of liquid in width and usually 3-4 in depth, you could dig a 7x7 wide hole at least 4 blocks deep and fill it with Tar to optimize the amount of Coal created. You can make it even deeper and use more than one Freeze Bombs on it too. Or you can create cataracts from Tar.
To fill a hole properly without leaving any empty spaces you might want to build bridges over the hole one after another and place the Tar on each block of the sides of the bridge, or create a "roof" where you can place the Tar on from down below.
After the Nodes have been created, Extractors of any kind can be applied to the Coal Node as usual to extract the Coal inside. Some players prefer to use up all of the basic and Advanced Extractors that they have found in Treasure Chests, but others will craft Super Extractors from the according rare Recipe that can only be obtained from the Thing, in order to receive the maximum output.
With update R26 in December 2015, the balance for creating infinite amounts of Coal (and Diamond Ore) was changed. "Coal Towers" that could be simply made from Ice before that date will now not produce Coal any longer, since Tar now flows significantly slower and fills one block after another with a new Tar-source each that does not cool down deep enough by being surrounded by Ice blocks. Tar can now only be made into Coal Nodes by using Freeze Bombs.
Since update R34 in September 2016 the crafting recipe for Freeze Bombs now requires to craft Iron Mining Cells and Armor-Piercing Bombs to be unlocked, while the Freeze Bombs themselves need to be made with Rambeau Tufts and Blizzard Chizzard Gizzards as ingredients.