Well first you need an appropriate field to store the user name or ID. I would use ID, up to you.
Then in in your form, you update the relevant field.
As mentioned you could just use the Environ as I did below
Code:
Private Sub Form_BeforeUpdate(Cancel As Integer)
If Me.NewRecord Then
Me.CreatedDate = Now()
Me.CreatedBy = Environ("username")
Else
Me.AmendedDate = Now()
Me.AmendedBy = Environ("username")
End If
End Sub
You use whatever you prefer.
Where I was using this code, no one would even know or want to spoof the username, so adequate for our purposes.
Regardless, the basics are you store the value you want and just set a field in the table/query.?