The easiest way to do that is by turning their anger towards a group people are already mistrustful of.
Two issue with this: firstly, this may not be true. It is equally easy (and perhaps more sensible) to turn the blame towards the elites - who have been in power and are the actual source of frustration for many people. Anger at elites would not be either unnecessarily damaging or unfair, given that it is often for their poor choices that the majority has to suffer.
The second response is that even if it is easiest, that does not necessarily mean that is the path taken by populists. This premise relies on the assumption that populists will do all they can to get elected and seize power, when in reality they often are motivated by an actual desire for change.