Let us choose at random a point form interval (0,1) and let the random variable X1 be equal to the number which corresponds to that point. Then choose a point at random from the interval (0,x1), where x1 is the experimental value of X1; and let the random variable X2 be equal to the number which corresponds to this point.
Make assumptions about the marginal pdf fx1(x1) and the conditional pdf fx2 I x1 (x2 I x1).