An oak armchair flatpack can be made at a workbench (oak workbench or better) in the workshop of a player-owned house. This requires 26 Construction, uses 3 oak planks, and gives 180 experience. The flatpack can then be used on the chair space hotspot in the Parlour, while building mode. This requires no Construction level and gives no experience.
The cost of making and selling an oak armchair flatpack from scratch gives a profit of: -1,388
After completing the hard Fremennik Tasks, 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.