Mishnah Berurah — משנה ברורה

Standing - That is to say, the wrapping and the blessing should both be done while standing. [Regarding] the wrapping, as it is written "for you,"... and we learn from here a gezeirah shava (a tradition of a known rule applying to a new case based upon an identical word or phrase in both cases) from Omer as it is also written "for you"... and in the case of Omer it's written: "from when you first put the sickle to the standing grain" and [the sages] expounded [the word בקמה (to the standing grain) to mean] "standing." And [regarding saying] the blessing [while standing], this is because all blessing on mitzvot should be done standing. And this is only l'chatchila (preferably), for after the fact [if it was not done standing] he has fulfilled his obligation for both (i.e. the wrapping and the blessing) in any position. Most of the Archronim agreed that preferably he should make the blessing before the wrapping, at the time that he's holding it in his hand and intends on wrapping it, for that is "immediately prior to doing it." And he shouldn't make the bracha while his טלית is still folded, for that is "prior to the prior".