I had the same trouble too. I had to practice though. MY HEART WOULD JUST BEAT REEEEAAALLL FAST when i began to start writing and the whole time I was writing on the cake!! i hated writing and still do!
If you ever have icing left over from a cake, keep it, practice on a plate or something you can scoop the icing off of the surface and start over again. Practice basic words like, Happy Birthday, Congratulations, Best Wishes, etc.
Another thing that worked for me is that as I got better , I had to thin my icing out a little more and use a smaller tip. i use to use a 3 , now #2 writes better for me...depending on what kind a contrast I am trying to get on the cake... and yep, write fluently and at a quick but steady pace, watch your icing as you write and the tip, sometimes you can tell when your icing is getting ready to clog up in your tip, almost like i can feel it a bit, squeeze it off in to a bowl, wipe the tip with a papertowel and keep writing...and funny thing, I hold my breath while i write, not intentionally, but it seems like when i do , I write better LOL.
One more thing I had started doing was, if I was trying to center a word, I would count the letters off , it there was a middle letter I would write that letter first in the center and then starting from the middle letter going to the left , i would finish the word backwards-dont know how old you are but kind like using the typewriter before computers and Word came on the scene- the counting off your letters technique.
But dont, worry, YOU WILL GET BETTER
..."If I can help somebody, as I travel along..then my living will not be in vain..."
..."If I can help somebody, as I travel along..then my living will not be in vain..."