you can shadow box however you want really, theres a million different ways to do it.
It can also be extremely tiring.
If you shadow box with 100% speed and effort, its very tiring, its something I do in my final preperations for a fight.
Either go hard with it for cardio.
go light for technique
mix and match
work your footwork.
Have fun with it. Just get out there and start doing it. once you start doing it, you will find yourself creating your own routine, doing your own things.
another thing I used to do, was sometimes on fridays, end of week training, if i was burnt out, i would just do a few light rounds of shadowboxing, few light rounds of bag, and call it a day.