Start with Left-Wing newsletters. Infiltrate churches. Get some socialists on AM Radio. Start a propaganda Leftist cable network. Create partisan think-tanks. This will be a multi-generational fight.
The Right Wing has it figured out. Democrats think they are showing up to a debate club when the other side has been out for blood for the last 50 years.
Energize the base, and the moderates will eventually fall in line (just look how small and ineffectual the "Never Trumpers" turned out to be). The base drives things now. You can't just fire up the base and then slap a Romney or Clinton on top and expect to win elections anymore.