Decide what kind of concentration you want. Tailor the game to what the child is learning. Sight words, for example, would be good on a concentration game for a kindergartner. Colors would be good for a preschooler. For an older child, have them match math problems with the answer. Personalization is key here.
Cut the card stock into 2-inch by 1-inch pieces if you have decided to use that instead of index cards. If you are using index cards, cut them in half. Be sure all pieces are the same color.
Make cards in sets of two that are identical. If you are making a color concentration game, make circles of each color. Remember to make two of each color so that the child can match them. If you are making sight words, write the same word twice.
Laminate the cards for posterity if you want them to be around for awhile. The laminated cards are much more durable.
Lay the cards out face down in rows of four or five. Have the child turn over one card then another. If it is a match she keeps the pair and goes again. If not, she turns them back over and play passes to the next person. The goal is to collect the most matches.