My favorite tools of the trade are Bristol Velum paper, H pencil, art eraser gum, India ink, alcohol based markers, and stamping inks. I personally buy big sheets of Bristol at my local store and cut them to size, but as you can see in the picture below, many cards come already cut in different textures, colors, and thickness.
I like using H pencils, because they do not leave dust behind, and when you erase your paper is clean. But any regular pencils like the ones you buy from school are probably fine. Because I was making a Halloween card, and I used a cute scrapbook Halloween paper, I decided to get my inspiration form on of the design of the paper and draw my cat to match. Unfortunately the picture is taken too close, so is hard to see.
Then I trace with India ink. I used to use only nibs and a plume, but I found these India ink pens and I love them, and they allow me to do a finer detail. Be careful which ink you buy. Not all calligraphy ink is India ink, and when you pass your eraser, or fill with color they will run. You could probably use a writer marker, but if you put a finish to your card it may also run.
India ink only takes minutes to dry, and then I erase my pencil marks and fill with color. My favorite is alcohol based markers, but whatever makes you happy! Aquarelle markers, student markers, colored pencil, water color. I have even used them with water soluble oils! I often mix techniques, and no matter what I use to fill in color, I always add finishing touches with colored pencils, and gel pens. Here is Texas some call colored pencils map pencils!
As a finishing touch I often use scrapbooking ink on my borders. I went crazy on this one with black and went all over it, and I personally think it was a mistake…. I went back on it with more colored pencil. It still looks a little dull, but is ok.
Then I cut an envelope to match the size of the ATC. You can add border and designs if you wish. I have these big scissors that are wonderful rounded edges. I am sure there are many punches that also deliver a similar effect. I made my own template, and below is a picture of it so you can use it!
Here is the template for the envelopes!