Mishnah Berurah — משנה ברורה

Buys a Tallit etc. - Don't ask [the following question]: Regarding other new vessels/garments, one needs to make the blessing [of שהחיינו] at the time of buying it, so why [here] did he write "and makes Tzitzit strings for it"? (ie. he says to say the שהחיינו only after affixing the strings). For one can say/answer that that is specifically when one buys a garment that is ready to be worn "as is" [in which case] one would make the blessing immediately. But if one buys a garment/cloth to make from it clothing, then one does not make the blessing even after he finished making it [but instead makes it] at the time that he wears it (for the first time). For [the blessing of שהחיינו] needs a designated time... either when you buy it or when you wear it. And here, it's not fitting to be worn right away for one needs to place the Tzitzit on it. Therefore, he is not able to bless on it at the time of acquiring it. And in order to also to fulfill the opinion of those who hold that one needs to make the שהחיינו blessing on the "doing" of the mitzvah for the first time, one should make the blessing at the time of attaching the Tzitzit and not wait until they wear it.