Prepare a set of cards before the lesson; I usually make simple pairs, but sets of 3 can also work quite well.
Give each pupil a card and ask them to find their partner(s). I deliberately choose very similar examples so that pupils have to think carefully about their answers rather than just looking for someone with "matching" numbers.
One of my favourite activities using Mix and Match is to support expanding and factorising quadratics. I picked examples differing only in signs, such as (x + 3)(x + 2), (x + 3)(x - 2) etc. Once pupils had found their partner, I asked each pair to find their partner pair, then group up into a family of 8. We then used these quadratic "families" as examples in later lessons when dealing with the inevitable issues with signs when factorising.