Hrmm.. I find that the melody I come up with on the guitar by itself is generally different to the one I come up with when I sing-a-long with the rhythm. I like to think of it as the singer's melody as compared with the musician's melody. You should be able to work out the "structural tones" no matter what the type of melody.

The structural tones are those notes that HAVE to be there. These structural tones usually fall on a dominant beat of a measure. All the other notes in between are just there for colour and to smooth out the melody.

Let's take the first line of Twinkle Twinkle:

Twink-le Twink-le Litt-le Star
C C G G A A G


The first line is how it's traditionally sung and taught to children. You could just play the C once and let it sound until you need to change to the G, then let the G sound until you change to the A and so on.. these are the Structural Tones of this melody.

Now to spice it up a little more:

Twink-le Twink-le Li-i-it-le Star
C C G G A B C B G


This is a very simple example of course.. but you can see that when I got to "Little" I naturally wanted to embellish it by adding a couple of extra notes in there. However, the structure of the melody is still intact... this is how you play a cover song in your own style.. you maintain the structure of the song but spice it up with your own style.