To create a vignetting tool :
Take a piece of think black card and fold it in half.
Then cut out a semi circular shape so that when you open the card up it is a even circle/oval shape
Place across the photo paper when you are exposing your print:
To create a softer effect, do the same as above, but when you're exposing the picture, move the black card slightly :
I personally like both effects. The first one shows more of the photograph, but the second one has a dreamier and softer effect to it.