I'm shocked more people aren't already better prepared for this, but it's actually the same thing we should be doing now, and continue to do if Biden actually wins:
Organize locally. Get hyperlocal. Organize your city, neighbors, block, building, workplace, and friend group. Establish mutual aid networks, take back city councils, county boards, and school boards from the stewards of landlords and big employers.
Establish protection for those who need it, and plant the crop of future national leaders who will do the long overdue work of putting government to work for the masses. Make it harder for any president or governor to exercise state violence by getting our people in prosecutors offices and judgeships.
The Democrats are not going to save us. Biden is just planning to go down with the ship. Absolutely vote for the lesser of two evils if you live somewhere where your vote matters, but recognize that the Democratic party is not an ally to the antifascist movement, because combating fascism conflicts with the interests of capital.
That can change, but until it does, don't put faith in them. Anything positive they do will be because power-players were replaced. And that starts in your own neighborhood.
Edit: Since this has been well received, I want to say a quick word on third parties: don't be afraid of them.
Without getting into a lot of details, there are a lot of myths about the risks that third parties present, but before we parrot all of that, we should just acknowledge that there is no harm in listening to their platforms. It doesn't make you a kook to hear ideas that often get suppressed by the mainstream, or to use third parties as a way to network and meet people. I think Jill Stein has a lot of useful things to say, and if a pollster asks me, I will gladly try to bump her numbers up so that she is harder to ignore. Don't be afraid to do that.