here's my starter set
so i recently got into atc's thanks to swap bot so here's some.
first you need a background it can be card stock, chipboard, or art matte materials. those should be cut to 3.5 x 2.5 the size of a trading card.
then you just decorate the front any way you like, collage, draw, paint, embellish, whatever! but keep in mind most people want them to fit into a card protector so you can store and protect them. and then enjoy your work!