how do you write a great song without screwing it up...
You have to be in the right mood. When I write, I have to be feeling a strong emotion for it to sound right. Also take your time, and if you are having troubles, work on it over a couple of days and eventually you will figure it all out. Maybe listen to some music that has to do with what you are writing about, might help a bit.
How to write a great song is to write it from yor heart what you are feeling what you have been threw.