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You just send steem not steem dollars to minnowbooster here on steemit. The amount of steem gets converted in steempower for one week. If you want it for 4 weeks you put in the memo field: "4". So that's it. The amount of steem transfered dictates the steempower you'd get. 75 steempower for 1 steem. I believe this is the rate :D

But this is not profitable at all. I don't even know if i'm going to recover my "investment".