The Zman for putting on the talis for ex. Is 6 am. Can you hold the talis over your head at 5:59:30 or before in anticipation of the 6:00 zman or by doing that have you already been yoytzei the mitzvah of להתעטף and should refrain from picking it up over your head before the zman.
You can hold it above your head with your hands supporting it, because that is not considered wearing it, however if you drape it over your head that would be considered wearing it. Even if it was draped over your head before the zman, you can still move the tzitzis around a little bit and then make the bracha.
Shulchan Aruch O:CH 4-10, M:B ibid 23, 18-10, Piskei Teshuvos 8-5.