Excel Macro: If the characters " XX " are in the middle of a string then split the string at that point

I'm trying to divide a column into two columns based on finding a specific characters in the string.

Specifically, if ColumnA contains the string " XX " then, copy all before " XX " in ColumnD, and copy " XX " & everything after to ColumnF.

Sub splitit()
Dim i As Long, l As Long
l = Cells(Rows.Count, 1).End(xlUp).Row
For i = l To 1 Step -1
    If InStr(Cells(i, 1).Value, celltxt, " XX ") Then
    Cells(i,4).Value = Left()
    Cells(i,6).Value = " XX " & Mid()
    End if
Next i
End Sub