You would give the HideThings an argument that specifies which workbook to operate on:
Sub HideThings(wb2WorkOn As Workbook)
With wb2WorkOn
    .Sheets("Fusionspapier").Visible = False
    .Sheets("Data").Visible = False
    .Sheets("Name").Columns("L:L").EntireColumn.Hidden = True
    .Sheets("Name").Columns("N:N").EntireColumn.Hidden = True
End With
End Sub
Your calling routine then calls the HideThings routine with wb as its argument:

'Some code which sets wb to a workbook
HideThings wb