The short answer is to add creamed honey to your liquid honey and it will serve as seed crystals and converting all of it to creamed honey. If you don’t already have some creamed honey you can take crystallized honey and grind it up into the smallest crystals possible. Either way you get your seed honey you need to mix it thoroughly with the rest of the honey and store it in a cool place.
I accidentally made it one year when I harvested honey from a hive that didn’t make it through the winter. There must have been some crystallized honey in it as it turned into something as smooth as butter and tasted amazing. Unfortunately it’s now all gone.
A Prof. Dyce from the University of Cornell did research on this to produce a more controlled process and his write-up can be found here.
Give it a try, if you don’t like the results you can always warm the honey back up enough to melt the crystals.