First off, let me say I am staining with left over coffee. You can color your piece with just about anything as long as the paper you are using will stand up to a bit of water. So you can you use watercolors of any type, paint, tea, wine, even turmeric made into a paste and brushed on.
I have two pieces here. One is a Zentangle® Tile and the other larger on is a piece of Canson watercolor cold press 140 lb paper.
*First step is to get the rubber cement onto the paper. This can be accomplished by dripping the rubber cement from the brush onto the paper. I have found if you remove the brush from the tin and immediately turn the brush upside down to sort of collect the cement onto the brush you will have a good amount to drip or *trail* onto your paper. You can also brush it on for larger negative spaces or use other appliances like a chopstick, fork, smaller painting brush etc. Once it is on the paper, let it dry completely.
You can even go back once it has dried a bit and add some more color to areas you would like darker.
*When this has dried completely you are ready to remove the rubber cement. You should be slow and deliberate with this step as paper is fragile and can rip. Although, it can add a "distressed" look to your piece if you desire. Two ways of going about this. One is to rub the cement off, although I have experienced more tears this way. The other is to start and edge and slowly pull the rubber cement off the tile.