I am in quilted postcard exchanges and the theme of one of the exchanges was tree bark.
This instructable is how I made my fabric postcards.
This particular postcard exchange had one requirement, that you make a 12 X 12 square of your bark and then cut it into six 4 x 6 postcards. There are 6 of us in this trade, so we mail 5 to the others and keep one for ourselves...ending up with 6 different bark postcards.

This is a rather artsy way to create bark, I wanted ragged edges and the light and dark values of the fabric to play the part rather than look exactly like tree bark.

This is my first instructable...be gentle!

Step 1: Starting the Background.

I started out with a 15" x 15" square of black fabric. I thought there would be some shrinkage due to the stitching so I should start out a little bigger than 12 x 12".

I chose a stabilizer for the back of the black fabric. This particular stabilizer tears away after stitching.

The stabilizer is 11 wide, so I use two pieces, overlapping a little.

I pin the corners and the overlap.

Step 2: Adding Texture to Black Background.

I wanted to attach some type of fiber yarn to the black fabric for a little texture when the black would show through. I have a special foot for my machine that helps guide the yarn and get stitched at the same time. That special sewing foot is actually for larger yarn but it will work for this.

These things are called your feed dogs.

They have to be lowered so they don't touch the fabric.

I am using "eyelash" yarn.

I threaded it through this foot following my manufacturers instructions.

I sewed wavy lines all over the black fabric.

Completed background.

Step 3: Adding Bark

On my machine I put a free-motion foot, also called a darning foot. I put some brown fabric under it so it showed up better on the camera. I don't want to sew a straight line.

I used wavy cuts of brown fabric, different widths. I place the darkest down first and as I layered to lighter colored browns for a look of depth. There isn't much thought going into the cut of the pieces or anything, and you can see that some of them are already kinda ragged because I was pulling and trying to fray the edges with my hands. I'll fray it more near the end in the dryer.
I don't know how much fabric it took. I used what was in my stash...I'm a quilter, I have a big stash of fabric.
I use my sewing machine to sew a squiggly line down the center of these bark pieces. Using the free-motion foot allows me to squiggle where ever I want.
I continue adding more layers of strips, sewing down the center of each one.

Step 4: Take Off the Stablizer.

View from the back and time to tear off the tear-away stabilizer.

Stabilizer off.

Now I wet the piece and stick it in the dryer with a towel to fray the raw edges of the fabric.

The third picture is all fluffed up.

Step 5:

I am going to make them into postcards, and I will want to sew around the edge of the postcard so I want the bark flattened a little to make the sewing easier. I spray a little water to help with the ironing. I use the point of the iron to go down the stitched line of each piece of bark, I'll fluff it back up again after the postcards are done.
I turn it over to the black fabric and use some starch on the black side to give it a little more stiffness.

Step 6: Making the Written Side of the Postcard With Card Stock.

When I prepare the address side of the postcard sometimes I use any white or light colored fabric for this step and I write the address and information on the fabric with a permanent pen; and other times I use fabric prepared to go through a home printer, purchased at most fabric stores, but this time I'm just going to use cardstock.

I pre-printed my card stock with the addresses of the ladies I'm trading cards with. Then I cut them into 4 x 6 postcard size. (didn't take picture of the 4x6 piece of card stock by itself).

In this example I covered the 'to' and 'from' parts of the postcard with little pieces of brown fabric to protect the innocent!

I place the postcard on the back of my bark piece (black fabric) and sew around the edges. Now is the time to make sure you have your feed dogs back in the up position.

I use a really long stitch when using cardstock because I don't want to perforate the cardstock too much.
I have a 'walking foot' on my machine, it assists the top layers in moving like the feed dogs do on the bottom side.

Before I get to the end I pull up the bobbin thread at the point I started sewing.

Then I pull up the bobbin thread from where I ended. I have four threads, 2 top threads (one from start, one from end) and 2 bobbin threads, and I tie these in a little knot.

Then using a rotary cutter and ruler I trim away the edges.

I continue sewing each 4x6 cardstock piece to the back until I've used all of them.

Step 7: All Done!

One single postcard.
All postcards together and some of the excess.
At the post office it will not fly to put only a postcard stamp on them; at a minimum they will need a regular letter stamp. There's lots of argument between fiberartists and the postoffice when trying to follow the rules and I've made up my mind that you just have to do whatever that particular postal employee wants you to do; it probably won't be the same the next time you go in if you get a different employee.
Most fabric postcards are pretty flat, but this one has lots of fluffy stuff, so I'll end up paying extra for "hand delivery" or something.