This is just a hypothetical question, I don't need a clutch kit.

But if someone had to install a full clutch kit, which is normally from the spring plate onward, what do you do about balance ?  How do you balance the kit or do you have to ?

From what I have read you have to balance the clutch.
Unless you put it back together without balancing the clutch. In that case you don't have to balance it.
Clear?
Seems they come balanced from the factory, but that is probably the last time it is done.
Plenty of posts here where the clutch has just been put back on with no ill effects.

This is usually no big deal but if you pull the old clutch of then take care to mark it but on several occasions I have washed the marks of or used a secondhand set of plates to no ill effect.
What I consider happens is that in the factory there are a team of gnomes that put clutch packs together and check the ballance and if ok mark them and fit them. The fixed and pressure plates are very obviously ballanced as an item to within say 5 gms and then are assembles and rechecked, if the 5g out all occurs at the same sopt then no go so try a different combo from the big box ot plates.
If the ballance is out and you get vibration then move one of the plates 120° to the rest.

Regarding the installation of the new clutch
Mark old clutch´s assembly regarding the output shaft
Take out the old clutch assembly
Find old assembly´s mass center´s radial position, so you  can refer it to the output shaft .
Assemble the new clutch, balance it and find it´s mass center´s radial possition
Install the new clutch assemble  with it´s mass centre at the old one´s radial position
Pray the gods for it not to vibrate.
Best regards
JAP