You have many ways to do it and Michelle and Elena already told you 2 of the ways you can do it.
Elena, isn't it too much work erasing? And well, Aren't you afraid of messing your original file?
I use PS-CS3 and I use the Rectangle Lasso Tool, I think it's the easiest way. Since I don't like to cut and then take then to another file and then come back to do another, I do this:
- select the letter I want with the Rectangle Lasso Tool
- CTRL+J click (with the alpha sheet layer selected) This duplicates what is in the selection.
I repeat until I have all the letters I need. Then I select their layers and then I take to my layout file, they go all together.
Hope it helps.