To minionize a monster, they get 1/4 hp, their attack dice and damage bonuses are halved (any result less than 1 is treated as 1), their CR stays the same but their cost from the encounter budget is 1/4. Thus, to make a "minion" version of a standard PfRPG ogre, you get:
Ogre Minion (CR 3, XP 200)
CE Large humanoid (giant) minion
Init –1; Senses darkvision 60 ft., low-light vision; Perception +2
AC 17, touch 8, flat-footed 17 (+4 armor, –1 Dex, +5 natural, –1 size)
hp 7 (4d8+12 / 4)
Fort +6, Ref +0, Will +3
Speed 30 ft. (40 ft. base)
Melee greatclub +7 (2d4+3)
Ranged javelin +1 (1d4+2)
Space 10 ft.; Reach 10 ft.
Abilities Str 21, Dex 8, Con 15, Int 6, Wis 10, Cha 7
Base Atk +3; CMB +9; CMD 18
Feats Iron Will, Toughness
Skills Climb +7, Perception +2
Regarding things like spells or spell-like abilities, special attacks like rend or swallow whole, and so on, the same basic rules apply. Save DCs don't change, but damage or other effects are halved. 1d4+1 Con drain, for example, would become 1d2+1 Con drain. Draining 1 level would stay 1 level, but draining 2 levels would become 1 level, and so forth.
This works best with soldier- and brute-type critters, who tend to have very simple stat blocks and are there mostly to fight-fight-fight -- really that's what minions are about anyway. Making something like a dragon or a mind flayer into minion not only doesn't work real well mechanically, it just doesn't make much sense thematically either. You could use this technique to make some sort of "glass cannon" creature I suppose ... I remember Young Sherlock Holmes had a very nifty "stained glass golem" for instance. The only problem is that with the reduced damage it's more like a "glass pea-shooter."
Whattya think, sirs?