Ok so here's how it goes!

Open a new canvas

open as layers and select your mask

go to colors and select invert colors

should look something like this ^ now

go to layers, transparency, color to alpha

make sure the color it's removing is "white" and click "ok"

now you have a transparent mask! yay!

Now we add the photo or paper!

open your photo or paper as a new layer

it might be necessary to resize the photo or paper so do that (make sure the chain is linked as to not distort the image!)

select your mask layer again

go to layers, transparency, and select alpha to selection

make your photo/paper layer active again

go to select and click invert

go to edit and click cut

make the mask layer invisible or just delete it entirely

& tada!!! should have something like this (saved as a .png for transparency)

Hope this helps! xoxo