From fast-2-fuse or timtex, cut 2.5" x 3.5" rectangles.
This is the size ATCs usually have.
Of course your postcards can be any measurement you like.
You may also purchase pre-cut fabric postcard interfacing - in this case, skip this step ;)

Side note: I've tried both interlinings, and for certain reasons I liked the fast-2-fuse better, but you may try yourself and find your fave.

Take a piece of fabric for your "main theme".

I used my selfmade lino-cut-stamp and silver pigment ink to stamp skulls on (pink) cotton fabric.
The pigment ink won't wash out after it's heat set, though I don't think someone will wash the cards...

Then I cut out the stamped skulls and free motion embroidered (or free motion appliquéd) them to precut woolfelt, which I used as background for my ATCs.

The woolfelt was cut bigger than the finished card.

Fun part: Cut away the excess fabric around your embroidery and fray the edges!!!

On your background fabric, mark the finished size of your ATC for placement of your embellishments.

EMBELLISH and have fun!

Use anything you have handy. Mix materials and try something new!
These cards are very simple, but you can imagine that one can really do a LOT with those... ^-^

Iron your ATC´s front to fast-2-fuse as indicated. The fast-2-fuse comes complete with instructions, so you just have to follow these.

Cut back woolfelt to finished size.

Iron ATC to uncut backside fabric (I suggest a white cotton if you plan to write on it later), cut backside fabric to finished size.

There's no need to take care of the grainline...

Using the satinstitch (a narrow zigzag) of your machine, sew around the edges one or more times, depending on the look you want.

Play with multi colored or variegating threads or just use another color for the second run.

Sign cards and you're finished!

Congrats. You now have another addiction! ^-^