How to execute a line-drawing subroutine/macro?

I am a complete novice in excel and VBA scripting. I need to know how to execute this subroutine:

Sub Connect(r1 As Range, r2 As Range) 
    ActiveSheet.Shapes.AddLine r1.Left + r1.Width / 2, r1.Top + r1.Height / 2, _ 
    r2.Left + r2.Width / 2, r2.Top + r2.Height / 2 
End Sub

called like:

Connect Range("A3"), Range("D7")

that I found in an excel forum. My intention is to eventually be able to select two cells and then run a sub to draw a line between them. I suppose this would mean having some sort of thing to extract the cell locations from the selected cells, and then running the aforementioned subroutine with the selected cell locations as inputs.