It's easy to make up a tune. The hard part is notating the timing of it - the lengths of the various notes.

Here's how I do it.

I get familiar with the words.

I sing a song to the words.

I enter the notes I just sang into Noteworthy Composer, using my MIDI controller (piano-style keyboard which connects to my PC).

I tell the computer to play back what I just entered. If I didn't get the timing right, I try again until it's right.

I put chords to it, trying to make a song which can be accompanied by guitar without making it sound like a military piece which is all 1-3-5-8.