Reverse stencils can be as simple as leaves resting on a sheet of paper over which you spray with color or done using masks. I die cut Reynolds Freezer paper using my Sizzix to create masks. It was easy to iron the mask onto the paper but don't use too hot an iron or it won't come off. (Ask me how I know!) Tim Holtz has some very cool commercially produced masks which can be used this way.
These freezer paper masks were applied after I had done waxpaper resist on white cardstock before adding color sprays. The wax paper was run through the Big Shot using a Cuttlebug embossing folder. The wax paper is placed between 2 sheets of cardstock and ironed until the wax melts. The wax then resists the spray ink creating an interesting pattern.
Any color sprays can be used. I made my own using Posh Impressions Inkabilities and small travel size spray bottles from Target. You need to add the inks to water until it reaches the strength you like. Trial and error on this one.
The bunnies made a cute background for an Easter card. I used The Cat's Pajamas stamps for the grass and sentiment. The egg is actually a small playing card I've had for some time.