Here's a simple card that was created by tearing an aperture in a piece of scrap paper to create a mask.
Secure the mask on to a piece of water colour paper.
Apply inks over the entire area the area starting on the scrap paper and working towards the centre.
Once the entire area is covered with ink, remove the mask and over stamp with black ink and a stamps of your choice.
I saw this card somewhere on the internet and rather liked it so thought I would have a go at reproducing it.