You open your signature file, copy it and close it.
Then you paste it on your image file.

Your signature should appear where you want it to be.
If it appears in the middle of the screen, it means that
the boundaries of the file have not been registered (your
image>edit>stroke... did not leave a 1 pixel border).

Flatten the layers (note that you will have two layers-
your background and your signature layer which need to merge)