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.

Because it is not any less [important] etc. - But one does not make the שהחיינו on the [actual] mitzvah [itself] since it's something that does not come from time to time (ie. periodically... like shofar or lulav etc.). According to this, if one made Tzitzit on a garment that he already had, even if [that garment] never had Tzitzit on it, he does not need to bless שהחיינו. And the same law applies if one made תפילין for himself, he does not bless שהחיינו. And see [further] in the Be'er Halacha.

At the time he wears - First [he makes] the bracha of להתעטף and afterwards שהחיינו [Pri Megadim and Derech Hachaim]