There is a great guide on blog.macsales.com that shows you how from scratch:
http://blog.macsales.com/15617-creating-your-own-fusion-drive
However what they have missed is that if you have a drive that already has existing data, you can still create a Fusion Drive on it! This is because the Fusion Drive creation process merges the drive's data. I have done this before on a Mac Mini to know. Just make sure you have backed up your data using something like Superduper to clone the drive. Time Machine backup is okay, but it is not the best backup (as it doesn't backup every single little thing unlike Superduper).
As for the best way to upgrade a Fusion system to Mavericks, my suggestion would be to start clean to get the best results. Backup your existing data via Superduper to a single USB drive/dock/Thunderbolt drive just in case things go wrong. For Thunderbolt I use a Seagate 3.5" Thunderbolt Adapter for Seagate Expansion drives (which is a huge asset in terms of saving time - 1 hour is usually more than enough to do a complete drive restore/clone). Once you've done this, boot from the external Thunderbolt/USB copy pressing the Option key and selecting the external drive so it boots into the Desktop. If you have an SSD installed for the Fusion, I would suggest you have it installed first before you try booting this way.
Once you have finished booting into OSX, you can use Disk Utility to format your existing main drive, then open Terminal and following the Macsales guide to create your Fusion drive. Once that is created you can then run the Mavericks installer, and point it to install onto the Fusion Drive you have just created.
Restart your machine using your new main drive and you will come across the Mavericks setup section to setup Mavericks on your Fusion Drive. Create a username which is different to the one you normally use ie. JaneSmith instead of JoeSmith, and finish the installer. Then with your USB/Thunderbolt drive plugged in, go to Utilities > Migration Assistant and ask to migrate data from an external drive. The program will migrate all the data and preferences from your old copy, and mark which softwares are incompatible. When it finishes, select Apple > Log Out, then log into your normal account ie. JoeSmith and go to System Preferences > User and delete the new user account you created for the install process ie. JaneSmith. When you restart the machine again it will look just like your old setup, except be running on the Fusion Drive.
