This is most easily done with foreign policies, as few Americans have first hand knowledge of what is actually happening. Information shows that Pearl Harbor was not the surprise attack that everyone thought it was, yet Roosevelt painted it as such to manipulate public opinion. Before Pearl Harbor few Americans wanted to go to war. They supported the Allie cause, and were willing to send aid but the majority was against declaring war. Roosevelt supported war and manipulated public opinion in his favor.
Journalists manipulate opinion as well.
Often they will work for capitalist corporations and are unlikely to report anything undermining the capitalist economic system. They will also report news that will get them the most readers or viewers. By only reporting the sensationalized news they do the public a disservice, leaving them in the dark regarding many important issues. Unequal resources result in unequal voices, just as unequal campaign financing results in unequal campaigns. Those in charge of providing the public with information will consider where their money is coming from before they say anything.