I'd start by looking up your local Dems party. Most towns and counties have their own Dem party who's job it is to get Dems elected locally. They themselves are elected by (registered) Dems in that town or county. They'll know who runs your state party (also elected by Dems in your state), and who from your state gets to vote for DNC (national) leadership. They'll also probably know how many people show up for these elections (it's usually like 10 people!) Then it's just a matter of organizing enough progressives to take over these local party positions and then take over the DNC... but, the good news is, progressives just took over the party leadership! At least the new Chair is relatively progressive, and was supported by Our Revolution.
On the one hand, it's super annoying that a small group of people makes all the decisions, but on the other hand, they are the people who show up month after month. You'll probably need to show up too, get to know who is progressive and who's a shill. Then make moves to replace the shills with progressives.
In Nashua, NH (the second biggest "city" in NH) we were able to completely replace the city Dems leadership with only 50 progressives showing up to vote
What we really need is a national org like Our Revolution or Move On or whoever, to make a concerted effort to plan out the takeover of every town/county/state Dems in every state. Each state should have a paid director plus staff who can map out all the leadership of all the smaller Dems parties in that state, rally progressive voters, and take the state party for progressives, town by town and then the state party all together.
All corporate shills really... And the oligarchy