I'm using a Christmas card template from Firefly Creative. Since this is a horizontal card I am going to increase the width to 12".
Then I am going to increase the canvas size, by going to Image>Canvas Size.
This is going to bring up a dialog box with shows the original dimensions at the top. Below are two boxes for changing the width and height of canvas.
You will put in your new values in these boxes. We don't want to lock the aspect ratio as we have already set the width the size we want. At the bottom is the placement. We'll get to that in a minute.
The little boxes with arrows in placement tells the program where you want what is on the canvas to go. They really are pretty self-explanatory as the up arrow places everything at the top, the down at the bottom, the center places everything in the center and so on and so forth. Sometimes you are not going to want them flush with the sides so on the right you can put in the coordinates.
I decided that I wanted everything at the bottom, so chose the down arrow.
Then selecting the background layer I flood filled it with black (or whatever color was used by the designer). And now from an 8x4 card, I have a 12x12 template for scrapbooking.
There are plenty of other ways of doing both conversions and you may find one that you like better. Experiment, check out the help on psp and do some searches. But most of all, have fun.