I think both Artful Codger's and Helen's approaches have the risk of being formulaic, but I can see the value of them if you are trying to encourage someone who lacks confidence. It gives you structure. Both methods are equally valid. Some people seem to like to work from the particular to the general - use details to build up a picture - others like to start with the big picture and develop the detail from there.

My own approach is somewhere between Deckman's and Stewart's. I like to read the words over, get a feel for them and let the words suggest both rhythm and melody. I use a recorder to capture the melody and I keep a little manuscript book handy to write down tunes I come up with.

There is a lot of value in Deckman's "Relax. Get ALL the fog and chitter chatter out of your head." I sometimes sit down with a recorder and improvise freely. Maybe start with a known tune or a part of a tune, then see where the music takes me. Sometimes, something worthwhile will come out of it, in which case I will go back and try to repeat it (you never do exactly, but you can usually capture the essence of it). Once I have it fixed, I write it down. Even if nothing comes out one of these musical "free association" sessions, I find it helps develop a sense and feel for melody so that when you set out to write something, you can usually produce something. I have written a number of tunes that way, including tunes to fit words.

Another useful thing is not to discard fragments of melody that you find worthwhile but at the time, seem to go nowhere. They are often still worth recording. I use Noteworthy to enter them into the computer and store them in a sub folder called "Fragments". If I am writing a tune, I can take a look at the fragments and see if I can do something with any of them.