A mask is basically a cookie cutter...you apply it to a colored layer or a paper and it gives you the shape of your mask.
Open up a new image in PSP and flood fill with any color.
Next go to Layers
Load/save Mask
Load mask from disc
When you click on Load mask from disc you will get a screen that looks like this:
Click
the picture in the upper left hand corner to change your mask. Once
you have it selectd click load. (make sure your settings are as above)
Now you should have an image like this on your image:
Next thing you want to do is Merge the group of layers that make up the mask.
Right Click on Group-raster 1>Merge>Merge Group
Now your mask in on one layer and you can resize it if needed.
I
ususally do my mask layer last in my siggies, but some people start
with the mask layer. Just do it however you are comfortable with.