A very simple way is to get the font you want, print out what you want to say in the exact size type.
Then... take some waxed paper or parchment paper and place on top of your type... make sure you tape it so a sudden breeze will not mess you up by moving the waxed paper.
Now, all you have to do is go over your saying with butter cream, royal icing, white or darker chocolate and/or icing of your choice. Place in the freezer when you are done for a few minutes to harden and you now have perfect letters.
Still another way is to make a mirror image of the saying and follow the same procedure as stated, but you can then take the waxed paper and apply it directly to the cake without touching it (you know... the old backwards written "ambulance" on the vehicle you see in your rear-view mirror written correctly.
Either way, you will have better looking writing instead of the shaky, messy-looking lettering you are used to seeing.
Hope that helps!