Using Enum

If you want to raise your own specific errors and have code to handle these errors you could put everything into a dedicated class.
Define all your errors using an Enum.

Public enCustomErrors 
    lFirstError = -10000000 + 1024 
    lSecondError 
    lThirdError 
End Enum 

Use the vbObjectError built-in constant to ensure the custom error codes do not overlap with reserved/existing error codes.
The Err.Raise method can also take custom Description and Source parameters so it worth having a description associated with each error.

Private Const sDescription_FirstError As String = "This is the first type of error." 
Private Const sDescription_SecondError As String = "This is the second type of error." 

Create a dedicated private method to raise each error:

Private Sub OnFirstError(ByVal source As String) 
    Err.Raise enCustomErrors.lFirstError, source, sDescription_FirstError 
End Sub 

Private Sub OnSecondError(ByVal source As String) 
    Err.Raise enCustomErrors.lSecondError, source, sDescription_SecondError 
End Sub 

To raise the error, just call the correspnding subroutine.

Public Sub DoSomething() 
   If (True) Then 
      Call OnFirstError("DoSomething") 
   End If 
End Sub