To sit, simply right click on an appropriate object when standing beside it.
By holding down Left Shift while pressing right click, you can also sit on building-layer stuff, but it can cause you to get stuck if you don't have something like Noclip Mode or a jump mod installed. You can disable this in the config.json file.
To add items to the list of sittable furniture, just add their names to the config file's SeatTypes (mind your quotes and commas!)
Technical
Requires SMAPI, uses Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.