Problem has been solved. I'll go ahead and post the solution at the bottom for the convenience of anyone who might have the same issue.
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Okay, so I got enough dwarves together to have a military. I went to the military screen and picked out the peasants, hitting enter to promote them and then A to activate. However, I wanted one dwarf in particular to be squad leader (I liked the squad name 'The Gilded Shadows' best), so I attempted to make her one by hitting enter to promote.
However, she simply got dropped underneath a civilian. When I tried to promote her again nothing happened, and when I activated her it drafted my civilian. I've tried messing around with the options, deactivating and promoting, activating and trying to rearrange, but it's just gotten worse. Now half of my civilians have been assigned squad members (though only a few civilians have actually been recruited). I gave up and deactivated everyone, but they're still messed up in the list.
So, end result is: I have 2 squads with the members I actually do want to be in the military, plus another squad consisting of my miners, with one squad leader and one dwarf under the OTHER member who is scheduled for execution anyway. How do I set the leader of a squad and assign other dwarves to that squad leader, please?
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Solution: After promoting a dwarf, you have to hit space to end the promotions. Then you can promote them again if they're in somebody's squad and you want them to be a squad leader. Promote all dwarves you want in your military, and then to assign them to squads, first hit enter next to someone who is in a squad, and enter again. To assign dwarves to his or her squad, select the dwarf you want as a member, then the squad leader. If the wrong dwarf is squad leader but all dwarves in the squad are correct, simply select one dwarf for promotion, then the current squad leader.