I started by using this code to copy a group of columns from one workbook to another. The columns should come from D-K on the source workbook and be inserted into the first empty columns in the target workbook.
 sourceSheet.Range("D:K").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(0, 1)
The above code worked correctly, but I wanted all of the columns I copied to be shifted down 1 row, so I modified the code to be this, which is probably a little awkward, but it works.
sourceSheet.Range("D1:D100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 1)
sourceSheet.Range("E1:E100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 2)
sourceSheet.Range("F1:F100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 3)
sourceSheet.Range("G1:G100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 4)
sourceSheet.Range("H1:H100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 5)
sourceSheet.Range("I1:I100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 6)
sourceSheet.Range("J1:J100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 7)
sourceSheet.Range("K1:K100").Copy Destination:=targetSheet.Cells(1, Columns.Count).End(xlToLeft).Offset(1, 8)
Now, all of those new columns I created have the first row empty, and I am trying to figure out how to put data into those specific cells. My goal is for the first row of these newly created columns to contain the date, as obtained from the filename of the source workbook. For now, I am trying to insert any data into these cells, but I cannot figure how to specify which cells I want.

Any advice would be appreciated.