Say the matching rule aloud
For the standard-deck format described in the linked Bicycle guide, a pair is two cards of the same rank. The suit does not have to match. When three of a rank are held, two can leave as a pair and one stays. Four of a rank can leave as two separate pairs.
Picture cards use their own labels
Our original mini deck has two copies of each of six pictures. Cat matches Cat, Kite matches Kite, and so on. A similar colour is not a match. Each card has a word as well as a picture so you can explain the choice without relying on colour.
Try a small teaching prompt
Place Cat, Cat and Fish face up. Ask which two go together, and then ask what remains. Switch the order and repeat only if the learner wants another turn. This is a suggested activity, not a test of ability.
Keep the formats separate
A picture pair and an ordinary playing-card pair follow different labels. Choose one deck and one matching rule before a round. The practice board is face up on purpose; a real Old Maid draw is made from an offered hidden hand.