As far as I can gather, here's what's up.
Minions are just like regular creatures, except they're pretty much only supposed to be melee critters. (DMG 54-55)
Given that, I'd guess you probably use the monster design steps (DMG 184), but don't apply any of the modifiers in the table "Monster Statistics by Role". Then, just give them one hit point.
Those guidelines seem to get you fairly close. Then I guess you just compare it to existing ones to get the feel you're looking for in the way of special powers(see step 7, monster design steps).
Still, it does seem odd that's the only one you have to dig for when the rest of the monster types are pretty clearly laid out. In a way, it seems like the "basic" monster is the baseline and all the other types (e.g. lurker, soldier, Elite, etc.) are add-ons from there.
--edit-- I should note that this seems to get in the ballpark for some existing minions. Just a quick spot-check, not a detailed study.