How do I set up GeyserMC so Bedrock players can join?

GeyserMC translates the Bedrock protocol to Java, so players on phones, tablets, consoles and the Windows version of Minecraft can join your Java server.

Make sure your server runs on Paper. You can set this in the Startup tab.
Download the Geyser plugin (Geyser-Spigot) from the official GeyserMC website and upload the .jar file to the plugins folder.
Restart the server so Geyser creates its configuration file.
Geyser needs its own UDP port. Check in the game panel which extra port (allocation) is assigned to your server. If you do not have one, open a support ticket and request an extra port.
Open plugins/Geyser-Spigot/config.yml and, under bedrock, set the port value to that extra port.
Restart the server once more.

Bedrock players add the server with the IP address of your server and the UDP port from step 5.

Floodgate

By default, Bedrock players also need a paid Java account. If you do not want that, additionally install the Floodgate plugin by the same developers. Bedrock players can then join with just their Microsoft account. In that case, set the auth-type value in the Geyser config to floodgate.

Keep in mind that some plugins do not work fully for Bedrock players, because their client displays certain features such as custom menus differently.