How to add GeyserMC to my minecraft server

As you said you downloaded the Geyser.jar file, first of all, you will not be able to add geyser to the your as you did not make a spigot server, and geyser is a spigot plugin.

First make a spigot server, if you don't know how to do that then look it up on youtube, it is pretty easy (except for the port-forwarding part).

Once you have installed a spigot server, run it once, then let all the folders generate. Then stop the server. Now where the server files are located, there you will be able to see a plugins folder. Simply put the geyser.jar file in the plugins folder. Run your server again, and It should work.

See this for more in detail clarity of the server installation