For me , most of the tunes come from simply listening. If I can get a tune rolling around in my head (or if it's one that I can whistle with my mouth) it will usually come out as a " reasonable facsimile" on the tinwhistle. My own interpretation takes over from there. I've heard some of the finest players run off great tunes , that I can't even begin to copy , because they haven't got the "hook" that will get me whistling along. Best advice : Start slowly and throw in the bits and pieces were you think they fit. Your perceptions will get better with practice.
If anything , I have to keep reminding myself that less is more , in terms of ornamentation. I get a bit carried away , some times !!
I think it was Amar who posted , a while back , that he finds himself whistling a tune with his mouth but including all of the cuts , rolls , etc . (the way he would on a whistle). I do the same thing. There's a point were the lilt and lift and the ornamentation reaches an almost "magical" quality on the whistle. That's what first attracted me to the whistle and it's what I constantly try to achieve. If it's not magical , it's not worth playing !!
IMO Just wish I could get it to work that way , more often !!
Have a Great Day and Fun Whistling !!