How to do the ‘layer mask’ technique
to use the layer mask technique, you first need to have two pictures that have the same background, so they will be easy to merge together – we want a seemless finish so it is important the the light is the same and that the shadows on the two photos dont clash
add a new layer to your ‘background’ image, this can be done automatically if you just drag and drop your other image onto the background image, as it creates a new layer and puts the image into that layer for you. once this is done, lower the opacity of the layer 1 photograph and the line it up with the background photo, where you want it to end up at the end of the process, after this put the opacity of the layer image back to full.
when this is done, and you are on the layer 1 picture, go to layer then find layer mask and click hide all. this means that your layer 1 photo will completely disappear, leaving only your background image on the screen.
after this select the paint tool, start at first with a low opacity and then paint over where your layer 1 image was. by doing this it reveals only the bits of the layer 1 photograph that you want. use a very low opacity at the edge of your image that joins to the other image as this gives a seemless connection and then slowly increase the opacity of the layer 1 image so that it becomes opaque when your reach your main subject – so it will look like it is part of the photograph.
after this, just simply crop the photo where you wish to create your final image, you may sometimes need to dodge or burn to make the lighting slightly better or create shadows that are not there because the pictures were not originally the same image.