The Lee Expander die that I have, came with two inserts - for two ranges of inside neck diameters. They do nothing inside the case neck - certainly do not "round it out". But they do create a flare at the very front end - so, if you have a good fit bullet seating die - that one bends that flair back to straight as the bullet slides home. But, on mine, the Lee Expander die only touches the very inside of the front rim - does not go down inside like a Lyman M-die. So if you have squashed, dented or otherwise out-of-round necks, need to run them through a sizing die - even with that sizing die's expander ball turned way out - do not need to resize the outside of case neck - the expander ball forced through from front will make the inside of that neck "round". The Expander die then creates a flair. And yes, sort of trial and error to get it set just right - and need your brass pretty much exactly the same length to get consistent flairs.
The one that I got came with instruction sheet - I just did what it said to do - seems to work to create a flair. With more experience at it, I preferred the Lyman M Die, since it creates a nice "seat" to hold the bullet before pushing it down. Then used my lathe to turn an insert for my Lee Expander Die - sort of made my own M-die, except it goes into a Lee Expander body - amazed even myself that I could do that!!!