Applying
To apply custom tile areas:
Have the farmer hold the flooring sample
Hold down the mod button (default Left Control)
Move the cursor to the tile to begin on
Press and hold the place button (default Mouse Left)
Drag the cursor to the tile to end on
Release the place button
There will be a translucent green rectangle over the affected tiles as you drag.
Removing
To remove a custom tile area:
Move the cursor to a tile in the custom tile area
Press the remove button (default Delete)
Ignoring
To ignore flooring placement restrictions, hold down the ignore button (default Left Shift) when placing.
This allows you to place flooring anywhere on any location.
Ignoring restrictions also ignores existing floor tiles when determining which tile to place - this is meaningful for flooring with multiple different tiles. When ignoring restrictions, tiles are chosen based on their position within the custom area.
Note that this can lead to non-floor tiles being floored, so you may have to floor an area piece-by-piece to avoid, for example, placing floor tiles on walls.
Notes
By default, flooring is not consumed when applied using this mod. You can change that in the config.
This mod doesn't mess with the game's existing flooring, so you can safely remove this mod without issues.
Config
You can customize this mod by editing the config file or using Generic Mod Config Menu.
Technical
Requires SMAPI.
Implements a Generic Mod Config Menu interface to change config settings in-game.
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.