After you have created planks using the Saw Bench, you have admittedly nice, but unfinished planks. Before you can use them for furniture, you have to refine them using the Table Press.
To place a plank on top, simply right click with an unrefined plank. Then, you'll go through two steps:
To refine, hold your left mouse button and move over the plank in the GUI. When everything is colored, the refinement will complete and the item transform into the next stage.
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| 4 planks blocks, 1 iron ingot