There are lists you can look up that let you know which followers can become stewards. Once you decide on which one you want, all you do is have them follow you to your homestead. Then they should automatically offer to be your steward. However, if your first steward was killed you may not be able to replace them. I found out the hard way that you only get one chance with them. :/ If you're on PC though I'm sure you could get around it. I don't have a link handy at the moment for a list, but just Google "what followers can become stewards in Skyrim Hearthfire" and it should come right up. Hope this helps.
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