I've been looking into it, and here's what I've come up with:
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Steps required to make a Minion out of a particular monster:
-Reduce monster's HP to 1.
-Set damage for attack equal to the minimum damage it can possibly deal. (i.e., as if all damage rolls were 1's.).
So, a Doppleganger Assassin (Level 8 Lurker) Minion would be:
HP: 1
Attack: Dagger +13 vs AC; Damage 6.
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Steps required to make a new Minion from scratch without any base monster:
-Set HP to 1.
-Generate the new creature using the rules on pg.184-185 of the DMG. The new creature should have AC, defenses, attack bonuses, etc, appropriate to its level.
-Set damage by using the "Damage By Level: Normal Damage Expressions" chart on pg.185 of the DMG. Use the "Low" column and replace all "die expressions" (eg, 1d6, 2d6, 1d10, etc, etc) with a 1.
So, an ENWorld Poster (Level 21 Lurker) Minion would be:
HP: 1
Attack: Flame and Troll +26 vs AC (Or +24 vs Will, depending); Damage 8.
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Reality-checking that against most of the Minions in the Monster Manual yielded good results. Those methods should produce balanced, playable Minions.