At the beginning of your end step, bolster X, where X is the number of tapped creatures you control. (Choose a creature with the least toughness among creatures you control and put X +1/+1 counters on it.)
11/24/2014 Count the number of tapped creatures you control as the ability resolves to determine the value of X.
11/24/2014 The creature you put the counters on doesn’t have to be one of the tapped creatures.
11/24/2014 Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain’s bolster ability.
11/24/2014 You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.