If I have a subset, how do I define an equivalence relation.

I understand it has to satisfy three properties:transitive, symmetric and reflexive, but I'm not sure how to give an explicit definition of the equivalence relation, for example on I where

$\displaystyle I=\{(x,y) : 0 \le x\le 1 \ \& \ 0 \le y \le 1\}$