An oak chair flatpack can be made at a workbench (wooden workbench or better) in the workshop of a player-owned house. This requires 19 Construction, uses 2 oak planks, and gives 120 experience. The flatpack can then be used on the chair space hotspot in the Parlour, while in building mode. This requires no Construction level and gives no experience.
The cost of making and selling an oak chair flatpack from scratch gives a profit of: -714
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 56 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.