Did you watch my video on the ICM? If not, take a look.
Suppose the players left are A, B, C, and D. If player A places second, the winner could be player B, player C, or player D. The chance to place second to player B is assumed to be the probability that B wins times the chance to win a tournament just between A, C, and D.
Probability(BA) = stack(B)/(total chips) x stack(A)/(stack(A)+stack(C)+stack(D)).
To get the chance that A is second, you add the probability of BA, CA, and DA. This is not something you should expect to do at the table. What you can do is use an ICM calculator like my program
ICM Explorer to learn to predict the level of risk aversion in common situations, and use that knowledge at the table.