How do you practice democracy in the US?

Mainly by voting, but also by lobbying your representatives (lobbying is a verb that means to talk to your representatives in an effort to persuade them. It does not always mean buying them dinner and vacations for under-the-table political favors), donating to think tanks for political action committees (PACs), volunteering in any of those, attending city council meetings, etc etc. There are tons of ways to get involved in democracy.