Why did Hashem instruct Moshe personally to speak to the wise-hearted men who manufactured the Begadim?
Ramban #1: Refer to 27:20:3:2*.
Ramban #2: He was instructing Moshe to inform the workers that they should make the Begadim with the Begadim's Kedushah in mind.
Seforno: He was instucting him to inform the workers to have in mind when manufacturing the Begadim, that they should sanctify him
What does the latter half of the Pasuk mean?
Rashi: It means that Aharon was to be sanctified as Kohen Gadol by wearing the Begadim 1 that are about to be listed, and that this, in turn, would enable him to serve as Kohen Gadol.
See also Pasuk 41, which implies that it was in conjunction with the anointing and the inaugural ceremony that the Begadim sanctified Aharon, as is described further in chapter 29.