Hi MiRu,
In short, MSU idea is about using superior force on an isolated enemy (be it unit, character, monster etc.) to inflict enough damage in relatively short time to break and/or destroy the enemy. It uses the fact that it has many units to control the movement phase to engineer multiple charges from many directions, preferably avoiding frontal attacks, to maximize the chances of success.
Because of that I prefer to allocate units to at least two groups: assault units and support units. Assault units are those who have high strength attacks, high weapon skill and can concentrate these attacks on relatively small frontage. In my current army these are Swordmasters, Lions, Dragon Princes and combat orientated characters.
Support units have very important role of delaying enemy units, destroying its support with shooting or in combat, basically, helping your assault troops not to be interrupted in their jobs. So reavers, archers, eagles, bolt throwers, they all have a lot to do to help control movement phase.
Sometimes the roles are not so easily separated. I often use Archers in combat because they provide ranks and can break steadfast and against lightly armored foes they can kill things too. Reavers can easier get to the flanks and their S4 on the charge may help. White Lions, on the other hand, due to their stubbornness can hold enemy regiment for a while (although they are too small for an anvil here) and potentially every unit can be sacrificed so that I can surround the enemy and execute that charge from many directions.
If you have time I wrote quite long article on how I see MSU working. It is a little outdated but the main principles are still the same:
https://warhammer-board.de/%5Bb%5DMSU%20Observations%5B/b%5D
I hope that helps!