"From the beginning of the sickle": (The intent is) when the sickle begins, and that its harvesting be only with the sickle.
"in the standing corn": that all be standing (i.e., that nothing has yet been harvested); that it be the first of all that is harvested.
I might think that he could harvest and bring (the omer), and (count) whenever he wished; it is, therefore, written (Vayikra, Ibid.) "And you shall count for yourselves from the morrow of the Sabbath" (i.e., from the morrow of the first day of Pesach).