הלכה על שמואל ב 18:23

[For] all Mitzvot, one blesses on them "over l'asiyatan" [Meaning "before/preceding". [As in the words of the verse] "And he passed (va'yaavor) the Cushite", whose interpretation is that he ran and preceded/passed before him]. Therefore one must make the blessing on the arm-tefillin after he places it on his bicep, before he tightens it. For the tightening is it's "doing" [Rem"a: Similarly, [the blessing] on the head-tefillin [must be made just] before tightening it on the head. Tur.]