This is not as easy as the guy makes it look. But it's not hard, really, it just will take some practice and trial and error.
Let's say you want to pick the dog out of one picture and put it in another, like this guy did. You trace around the dog with the lasso tool. Trace loosely, I mean you don't care if you get some of the background with the dog, just as long as you get the whole dog.
Now you do a Copy and Paste. This takes the part you traced and makes it a new layer. The pic looks the same but if you turn off the background layer now (make it invisible) you will see just the layer with the dog.
Okay, now you want to 'trim' around the dog with the eraser tool. This is a little fussy, but it's not as hard as it sounds. Make the dog layer the active layer. You can zoom way in and make the eraser very small and get it right down to the pixel. There are other ways to do this but this is how I do it.
Now to move it to another picture you can open both pictures, grab the dog with the 'move' tool and just drag it to the other picture. It becomes its own layer in that picture too.
Now you want to use 'transform' to size and move the dog.
It might also be nice to re-adjust brightness, contrast and levels for the new layer, to make it match the new background.
Finally, you 'flatten image' and it puts all the layers together into a new pic.
I do this all the time to put one person's head on another person's body (fun with party pictures!)
If you do it a few times you will get the hang of it.
