Once the data-pack is installed, you can craft a "Place Chair" item using the following recipe(you can use any planks):

The output will look like a green book, but when you take it, it will turn into a spawn egg called "Place Chair".

When you place it, a red cube appears in that spot, that's an un-set chair. To set it, just place a slab/stair in the same place.
If you place anything other than a slab/stair, the spawn egg will drop so you can place it somewhere else.
It will also drop if you don't place any slab/stair within 3 minutes.

To sit on a chair, right-click the part of the block highlighted on the following image: