n Photoshop or Elements... If you go to the top tabs and click on:
File
From there you will see a tab that reads:
New File
A box pops open and choose the meatric system
Choose Inches:
In the boxes enter your measurments:
5x7 or 4x6  what ever sz you want your card or invite to be.
Resolution should be between:
250 - 450
Colormode:
RGB
Background:
Transparent
So it should look like this:
 Once the base is created you can open up the digital file of the invite and drag it to the base you created and stretch it. It should look like the image below.  Then you can go to your layers tab and flatten the 2 layers together.