If none of the below helps, please report in the comments and include your seed, platform and version, and coordinates.

If your in-game world doesn't match the map here at all, it's most likely due to one of the following:

you entered the wrong seed or coordinates

you selected the wrong edition or version (the one used to generate the region you look at)

you used mods or resource packs that affect world generation

If you started your world on an older version, you must switch between the versions in the app. Always select the version that was used to generate the chunks you're looking at.

By default, the coastlines for versions 1.18 and above don't match up perfectly. This is not a bug, but rather because the map shows the underlying biomes instead of the exact terrain. Near coasts, ocean and river biomes often have land, while land biomes can be underwater.

You can enable the "terrain" option below the map to automatically adjust the colors of such mismatches.

For technical reasons, you need to know the seed of your world to use Biome Finder, unless, of course, you want to find a seed for a new world. If you're playing SSP, the app is able to fetch the seed from your savegame. Alternatively, you can use the /seed command ingame. In SMP, you can use the same command if you have sufficient rights. Otherwise, however, you're dependent on the server owner, who started the world and has access to the savegame and config files.

This app uses some relatively new web technologies. As a result, some features may be disabled for older browsers. I recommend using the latest version of a major browser such as Chrome, Firefox, Edge or Safari.

Mouse

Drag: move the map

Mouse wheel: adjust zoom level

Right-click: set a red pin and fill in the coordinate inputs below the map

Double click: toggle expanded map view

Keyboard

Arrow keys: move the map (mouse cursor must be over the map)

R key: randomize seed (mouse cursor must be over the map)

Escape key: disable expanded view

Touchscreen

Drag: move the map

Tap: show coordinates below the map

Pinch: adjust zoom level

Tap and hold: set a red pin and fill in the coordinate inputs below the map

Double tap: toggle expanded map view

The first thing you should do is select a seed and version. You can either type it in manually, or you can load it from your savegame. The latter can be done by clicking on "Load from Save..." and selecting your level.dat, or by drag&dropping the level.dat file into your browser window. Level.dat is a small file located in the folder of every Minecraft savegame. You can find the savegames in the saves folder of your Minecraft installation. On Windows you can use %appdata%\.minecraft\saves to get to that folder.

You should also know that a seed is always a number (up to around 20 digits). If you type in anything else (like letters), it will be converted to a number. The app does this the same way Minecraft does, so it's safe to use letters (and other characters) as well.

Thanks to Earthcomputer for his work on bedrockified, which helped improve support for Bedrock Edition.

Thanks to amidst for providing biome colors.