what is the key step of "merging" two images?

[kenny1999]

(05-14-2021, 01:34 PM)Krikor Wrote:

(05-14-2021, 01:02 PM)kenny1999 Wrote: I'd like to cut a face of someone and paste onto another, they have different skin colors
of course. After selecting and cutting the face, what is the next step to do? 
Would you please highlight me the key steps I'll probably need?

Among the possible technical options, one is shown in the video: https://www.youtube.com/watch?v=2e6FikWMkaQ

I tried to follow the steps of the video but at the point of eraser, the eraser doesn't erase.
then I am stuck at this step, what is the problem?? status said the layer is "locked", but 
I haven't done anything