The dotted lines only show up inside Gimp, not in your finished image. Generally they mark a layer boundary but there are others showing selections for example. You can turn them off from the view menu but my advice is leave them showing.
To save a lot of writing this is a 2 and a half minute demo. https://youtu.be/XJ7MR05ieA8 you might get some hints from it. There must be any number of youtube videos around on the subject. Do not worry about my Gimp layout - like that so easy to see in a video. Make sure you know where the layers dock is and the toolbox / tool options.
This is using two well matched images. If your two photos have different lighting to match or the scale is different then it can be much more complicated.
To save a lot of writing this is a 2 and a half minute demo. https://youtu.be/XJ7MR05ieA8 you might get some hints from it. There must be any number of youtube videos around on the subject. Do not worry about my Gimp layout - like that so easy to see in a video. Make sure you know where the layers dock is and the toolbox / tool options.
This is using two well matched images. If your two photos have different lighting to match or the scale is different then it can be much more complicated.