with ActiveCell.Borders
.Color = RGB(0,0,0)
.Item(xlEdgeTop).Weight = xlThick
.Item(xlEdgeLeft).Weight = xlThick
.Item(xlEdgeBottom).Weight = xlThick
.Item(xlEdgeRight).Weight = xlThick
End with
for specific range :
Range("a1", "b10").Borders.Weight = xlThick