ONE SUBJECT, TWO OR MORE TIMES, IN ONE PHOTO!
Here's how you do it. It's a lot easier than you may think!
The key to making this work is starting with a good series of photos. You should choose a photo with a background free of clutter. By this I mean, no distractions in the background. This makes it easier on you and is a lot less work. In addition, I try to stand in the same spot, so my background doesn't change much. And finally, when you shoot, shoot with your subject off center so you have room to add him (her) over and over again!
So, to begin, I selected my background photo. As I said, I chose one that was off center left with a lot of background. However, there wasn't enough room, so I copied a portion of the photo by dragging out my marquee tool and duplicating the selection [Ctrl + J (Mac: Cmd +J)]. Then I just slid the new layer into place to the right. I linked the two layers and merged them. However, you can see a seam:
I'm sure there are several ways to correct this, but I use the clone tool.
For this particular photo, my setting were:
However, you may have to adjust yours depending on your photo, how closely zoomed in you are, etc. You just have to play with it.
To use the clone too, you just Alt + Click on a section of the photo that you want to copy and then Click on the area you want to change. I use a hardness of 0 because I want my edges to blend in better.
Here is what my seam looked like after I was done. Not perfect, but certainly good enough for me!
Next, I went to the photos I was adding and used my lasso tool with a feather set to 10 pixels. Anti-aliased was checked.
I made a fairly tight selection around my subject and plopped him on top of the background photo:
I applied a layer mask to that photo and with a soft (0% Hardness) black brush, "painted" away the parts of the background that were too harsh. As I got closer to the body, I increased the hardness to remove the "halo" that remained. If I made a mistake, I painted it back on with white and tried again. Ah! The beauty of layer masks.
Once I was happy, I repeated the same steps with my third photo. And voila! The finished photo: