>>1492686Layer -> "Add Alpha Channel" (this is transparency)
add a new layer below the current layer with the flag in it
you want to very carefully draw a freehand selection around the shirt only (NOT including the hand) and delete that part of the layer so that the alpha channel (transparency) shows through, and through there, the flag.
then just move the flag around on the bottom layer until it lines up right
something like this
https://youtu.be/watch?v=z7dy7AXk65M 
