Hitler understood that previous far right movements had got nowhere because they were seen as representing the interests of a social elite. He knew that if the Nazis were to succeed, they had to reach out to the masses, to appeal to the broad swathe of middle and working class voters who had always been intensely hostile to the Weimar Republic.
The Nazis did this primarily by playing on the fears and concerns of millions of Germans. The country had been in the grip of a major economic crisis since the onset of the Great Depression. It seemed to many that the old ways of doing things had failed miserably. Germany was an economic basket case, with mass unemployment and hyperinflation plunging millions into poverty and despair. The Nazis were able to exploit widespread disillusion with the existing political and economic system and put forward a radical alternative.
They were always deliberately vague with their plans, making generalized promises which conspicuously lacked detail. But Hitler wasn't interested in the minutiae of policy; he was much more interested in putting forward powerful propaganda messages that could attract the support of the masses. As well as keeping their message simplistic and vague, the Nazis cynically tried to be all things to all men. Whatever your social group, whatever your occupation, wherever you lived, be it town or country, the Nazis had a policy just for you.
Although the Nazi cause did become a mass movement, it's important to acknowledge that they never won a majority of seats or votes in a free and fair election. Turning Nazism into a mass movement may have helped them get a foot in the door of power, but it would take shabby backstairs, intrigue, and a fatal miscalculation by Hitler's political rivals before the Nazis were able to take over Germany completely.