Such assignment depends on the available random number table, the population size, and the distribution you're trying to achieve. Even within a specified set of parameters, there are generally a number of ways you can do it.
Suppose you want uniformly distributed numbers in the range 0–20. You can pick 2 (pre-selected) digits from numbers chosen from a random number table. If those digits do not form a number in the range 0–20, throw away that random number and try again.