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It's quite easy, you post a few blogs a week, they make .50 cents each, you reply to 5/10 posts a day and make maybe $1! Hehe

just replying to someone else's post makes more SD?

Well it depends who they are of course, they need to have some Steem Power in the bank. I find engaging with anyone who's written something worth reading more interesting, and currently, more profitable.

and it doesn't need to get upvoted?

Yes, preferably by someone with plenty of SP. but any upvote is good to have as they will boost this number at the side of your name = more respect?!