Stop the server
Download pluginsYou need both:
Geyser.jar
Floodgate.jar
Choose for the right Version!
(check what version your server is on)

we suggest to download the plugins directly from https://geysermc.org/download
Upload pluginsUpload both files to:
/plugins

After the Files are uploaded, start the server and check your Console if floodgate and Geyser got succesfully enabled:

Your Server is now using Geyser and floodgate!