Open your program and open the paper you wish to use. I am using Ziggy Fan Scraps from her "Forever Love" Twilight inspired FTU scrapkit (so you can see the paper).
Here is the paper that I plan to use.
Now, I want to go to "Effects" in my toolbar and scan down in the drop down menu to find the "Alpha Mask" plug-in that I have recently installed.
The drop down menu will disappear and this box will show up. Click "Browse" and locate where you have saved your masks.
Locate which mask you wish to use. I'm using WSL #84 (I believe her correct name is "WeeScottLass"). This is so you can see what happens later. Click "open" once you've highlighted your chosen mask.
Next, you'll see the image location pop up in "Mask File". Just click okay once you've entered your mask location.
Once you click okay, you're paper will be transformed into a mask!
Or, if you are just starting out, you can save this as a *psd to use as a base of your newly started tag. All you have to do, once saved as *psd, is to add a white background under it and then start adding your layers over the mask.