The size of 2.5" x 3.5" seems to be the only requirement. And I think that the whole thing brings back nostalgia of when I used to have trading cards as a child. Did you? I remember many a happy hour sitting on the front "stoop" of our house in Des Plaines, IL trading "horses" for " gardens" or "flowers" or much-prized "dog" cards. These were dutifully kept in old cigar boxes and sorted by the photo on the front. "I'll give you 3 flowers for that golden retriever" or something like that. Sometimes we brought them to school and traded or just admired them at recess or lunch hour. This may have been a regional tradition as some people give me an odd look. This would have been in about 1949.
So now I am reading a little about the how-to's which of course are totally open ended. I went to the Strathmore site...and found some how to's there. And I went to a WikiHow site for some info as well. The latter even had a short video of someone making one with collage materials. Anything goes. These little doodling ones to start with were done with the idea that they might be fun for a "starter" project for our Fine Arts open house this Friday. I used watercolor pencils (Derwent), watercolors, permanent ink pens. We could start our own trade here in my park in FL or we could find other artists to trade with. If anyone has a good spot to go for trading let me know.