I wanted to post this here because a lot of people elsewhere were always interested in this very simple (yet time consuming) process.
I'm sure you can mimic this with digital, as you can watercolors, oils, etc... but I've been doing traditional art since I was 2 (i'll be 27 soon, so it's been a while!) so I'm a bit biased when it comes to prefering the 'real' stuff.
this is a material that gives your work smell for a few weeks plus it's fun and can be messy if you bump your work space.
There is no right or wrong to this, accidents are GOOD.
What you need to do to prepair
1. Large work space that can be free for a WEEK or more.
2. Garbage bag. what ever size or color. size depends on your piece of paper. you want it to have a 'protective edge' so that the tea doesn't drop off onto the floor or table. you WANT it to puddle under the paper.
3. paper. I use watercolor paper for this. the good solid heavy stuff. I'm working on a paper illustration board right now, but I'm not sure if I like that better. It warps a bit too much.
4. tea or coffee. Tea stains light. Flavored tea sometimes does leave behind a slight tint. coffee is a nice rich color.
Get a container. Cup/teacup/bucket/bottle. Fill it with hot water (brews faster). Place what ever ammount of tea you want in there, or save the left overs of your morning coffee that has NOT been sugared and creamed. I start off with 3-4 bags in a normal drinking cup. This produces a mid-tone stain. Let it brew for how ever long you want. save the tea bags, you'll want to refill again, most likely.
While tea is brewing:
Rip garbage bag down one side (the kind I use were made from two plastic sheets, then melted together at edges and bottom). rip open bottom.
Lay it down on the table, leave wrinkles if you want, or pull taunt and tape. I prefer the wrinkles
Now, how I usually work is a blank piece of paper, then an image'll pull out of it as I work on it after it's all dried. (like looking at clouds, sometimes you 'see' something). The paper gets laid down WORKING side down. the 'wrong' side is where I pour my liquids. But, keep in mind, you might like the wrong side better in the end.
Why? Because over time, the tea will seap through the paper, as well as puddle around and fall off the edge of the paper (hense why bag needs to be bigger, this way it goes and collects underneath). smooth bags won't leave any creases in the staining, but wrinkled bags, where ever the tea can't collect, you'll have a lighter color'd tea.
Pour the whole cup onto the paper, smooth it down so that the whole paper is covered. Let it soak. Repeat as often as you want, though I usually let it dry a bit before adding more. Once the whole paper is covered, you can start pouring tea more on one side rather than the other. My newest piece I've got drying downstairs was done in this fashion because I already have a painting in mind and it needed one side to be darker.
I will also sometimes flip the paper around and stain both sides, then flip it back a day or two later.
It'll look like this:


Those pictures were taken a week into this process. It's a layer of almost dry tea, with about a cup of coffee for the wet spots.
I use my kitchen table since it's only there for storage anyways ;) My fiance and I don't use it for eating. But, since we walk past it every day, the table moves slightly, shaking the tea this way and that, then we leave for the day and it settles, drying where it had moved on it's own.
I came across this process in college for advanced drawing. We were making collages and the majority of it could not involve drawing. I soaked paper in a bin (about 2ftx3ft). I'd soak for a day, take it out, let it fully dry, soak for a day or two, pull out, etc etc etc. It turned the paper into this leathery texture. I did this for a [i]month[/i]

this is only one half of the picture and it's not really that yellow (blame walmart's lovely photo center for that). My father took this 4ft long collage of paper dolls in victorian dresses away from me and it's hanging in my parent's living room 20hour drive away. All those wrinkles came from the teaing where the paper got soft and crinkled a bit, then dried and the crinkles remained.
*****I HIGHLY RECOMMEND! That if you work with tea for longer than a week, that you dump out any tea you've been using and make new batches. Tea will rot if you leave it in water for too long and smell DISGUSTING.
I also do not know how long tea/coffee will last, but I'm guessing it's alright. That egypt collage was made back in 2000 and it's still fine.
Try to stay away from tea that's not 100% natural.
For painting on tea, I prefer using watercolor, acrylic doesn't sit too well on top of it. Inks work as well, as does drawing on top of it with pencil/pen.
here are some more examples of what it does:


(lighter areas are where the bag touched the underside of the paper, and the darker is where the tea was allowed to pool. some of the tea that soaked through leaves marks as well on the underside.)

(this was painting with tea w/brushes, then mixed black ink or white into it for shades)
it's pretty much the same with tea as it is with how i paint.
if you leave it undisturbed, the tea will leave lovely edges as it dries, pulling back to sometimes leave rings, etc.
Remember, this can take well over a week. Pouring, letting set, etc can be a day or two between. I tend to use a LOT of liquids, so the drying can take -days-