use the magic wand and lasso tool (feather 2x) to make your selection. Then cut. Thats the easy part.
The hard part is filling in the empty space and making the background look normal.
Here is an old photo of my ol ladys dad and grandparents.. The bride, now Ex-Wife was right in the middle of the photo..
I took out the Ex Wife and moved the grandad closer to the groom.. it looks like they were standing next to each other but they werent..
I still never got the curtains right.. if you look close they are still a bit messed up but good enough