You don't need to go shouting it, but if someone starts talking politics at you, fucking own it. Some coworker is like "trump sucks", say "yeah I know, I'm a communist". Your grandpa says "trump rules", say "no he sucks ass, I'm a communist". You're on a date and they ask who you're voting for? Say "I'm a communist". Cashier asks would you like change? "Yes, I am a communist".
Be open about your politics and lay claim to the title. Be a communist.
Well, I don't really see these people too often these days, so I'm not really going for conversion. Plus, the conversation starts with them seeking to understand my views, and typically becomes a debate when they take issue with something I say. When I'm going for conversion I typically don't come right out and say I'm a communist; instead I just make individual points about how capitalism is fucked up and let things evolve from there (this worked on at least one co-worker).