Frankfurt Airport to Cologne

There are frequent high speed trains that take you from Frankfurt airport to Cologne in about an hour. The question is about buying a ticket, which I recommend doing in advance. You can get a cheap Sparpreis ticket but that is only valid on the departure you select, or you can buy a flexible ticket that costs more but allows you to take any train you like to Cologne.

You can get a train direct from the airport, 2 or 3 every hour.
I'd bookmark the website to familiarize yourself with the schedule.
https://int.bahn.de/en/

I'd wait until you arrive to buy the ticket which you can easily do from a kiosk machine or a live person.

The fast trains do it in under an hour. Some call at Koeln Deutz/Messe, directly across the Rhine from Cologne and you just take a short S-Bahn over the bridge to the main station.

Another option is to take the scenic route along the Rhine and you can see the gorge and all the castles. The direct trains go at 58 past the odd hours (9:58, 11:58, etc.) and take 2 hours to get to Cologne from the airport Fernbahnhof. At other times you take an S-Bahn to Mainz from the airport Regionalbahnhof and change to a train to Cologne.

Try to book a. ticket to Cologne, not Frankfurt. You will have the train ride include.

That would be airplane ticket to Cologne Hbf,. The Cologne Hauptbahnhof airport code is QKL. If you just search a ticket to Cologne, you will probably get an air route to Cologne/Bonn Airport CGN.