I am assuming you are talking about doing this programmatically.  If you are using .NET 2.0, this is much easier using the MACL and the ObjectSecurity property on the particular objects you want.  If you are using .NET 1.1, it is more difficult.  The easiest way I have found of doing this is to essentially first inspect the ACL before the change, then make the change in the UI, then inspect the object again and see the differences.  We have some sample code listed at the top of this forum that shows how to enumerate the permissions (Ch. 8 I think) on an object and will be a good start for you.