MB 1: If one bought a talis, etc. - Do not object that on new clothes one needs to bless when he _buys_ the clothing, so why then did the Shulhan Aruch write "and put tzitzis on it"? Our answer is that only when one buys a garment which is fitting to wear as it is, then he blesses immediately, but if he bought fabric in order to make clothing from it, then he should not say this blessing even after it is ready, but only when he puts it on [for the first time]. This is true because we require a distinct, well-defined time [of event or action] - either the time of purchase or that of first wearing [vs. when he finishes preparing a garment, which is the end of a process spread over a period of time, but is not itself an action]. Here, the talis is not fit to wear immediately because he still must put tzitzis on it, so he cannot say this blessing on it at the time of purchase. In order to comply also with the opinion of those who say that one should bless "Who has kept us alive" when he does a mitzvah for the first time, he should bless when he attaches the tzitzis and not wait until he first puts it on.
MB 2: As the talis is not inferior, etc. - But he does not bless "Who has kept us alive" on the mitzvah [alone, but only because it is also a new garment], as this mitzvah does not come at designated times [vs. a mitzvah like blowing the shofar, which on the one hand is not performed frequently and on the other, is performed at a designated time (Rosh HaShanah) so that it makes sense to say this blessing, which ends "who has brought us to this time"]. Thus, if one put tzitzis on a garment which he already had, even if it never previously had tzitzis, he does not bless "Who has kept us alive" [as it is not a new garment]. This is also the law if he made tfilin [phylactories] for himself, he does not bless "Who has kept us alive" [as we do not say this blessing on the mitzvah alone]. And see in the Biur Halacha.
MB 3: When he wraps himself in it for the first time - First he blesses to "[Who has commanded us] to enwrap in tzitzis" and then "Who has kept us alive".