Mishnah Berurah — משנה ברורה

One should not make the blessing on it - Even if only the inserting of the strings into the [hole in the] corner was done not for the sake of the mitzvah, and certainly [the same applies] if he made the top (ie. first) knot not for the sake of the mitzvah. For Rashi, the Rosh, and Tosefot all hold that on a Torah level the affixing [of the ציצית onto the garment via the holes] also requires [intent to do it] for the sake of the mitzvah. And the verse that excludes a gentile is needed [to teach us] that [even in the case of] a Jew standing over the gentile and teaching him to do it for the sake of the mitzvah [is invalid]. Therefore, one must be careful optimally, before affixing [the ציצית] to the garment to say out loud, explicitly, that he is affixing all these ציצית [strings] for the sake of the [mitzvah of] ציצית. And [this is] like what we wrote above in siman 11 regarding the spinning and interweaving, and see further in Biur Halacha.