I think that making a line with a little ruler or making the letters first with a toothpick is a good idea so that the writing isn't crooked. Also, planning where the letters will be so that your words don't go right off the cake. When piping let the icing "fall" onto the cake. Keep in mind... when writing with a pen, your fingers do the work, when writing with icing, your wrist/arm does the work. And practice first on some wax or parchment paper. Hope that helps you out a bit.