GIMP photo editting - How do I make a selection blend in wit

[Brice]

i think you have the right idea, first create duplicate of background, then by copying a section of grass, and pasting over top the girl in new layer, try that, save layer then on new layer try, copy the girl and grass around her equal in proportions and use clone stamp tool to copy pieces of grass near by and stamp over girl , then smudge edges a little with convolve droplet,
. .