“Cross over.” Since the Jordan was at flood stage, and the priests were on the east side, standing where their feet had touched the water, that these men might be carrying these stones a mile or even more.
“in front of.” The Hebrew word “before” has the meaning of “in front of,” and also “in the presence of.” These men had to pass in front of the ark to get the stones, and were “in the presence of” Yahweh as He held back the Jordan River.
“the midst of the Jordan.” That is, into the dry river bed.
“each of you take up a stone onto your shoulder.” This gives some indication as to the size of the stones set up as a memorial. They were large enough that they had to be hoisted up onto the shoulder, but small enough that one man could carry one of them.