You earn it by being a consistent and good zapper. You need to zap a certain amount a day to keep it. If you skip a day you will lose it. If you scroll over someone's hat it will tell you how many days streak they have had the hat for.

If you don't have enough sats to zap to keep your hat then create a good post and people will likely zap you. Zap them back and respond to their comments and you will probably get more zaps. You can deposit sats in the your SN lightning wallet but if you engage and create good posts and thoughtful comments you probably don't have to because people will reward you with zaps.