You're grossly misunderstanding the question. I mean what are the Nazis actually going to do?
Well, that was what the Nazis in charge of drafting postwar plans for the East planned to do. How well their plans would succeed, and how they would change when faced with reality, is another matter entirely.
For instance, IMO their expectation they would be able to refill everything east of Germany with German settlers was widely optimistic, even if they'd (and they would) push strong natalist policies.
In this circumstance, how do you decide which Ukrainians are going to be assimilated?
Well, they had part of their civil service charged to deal with such matters, e.g. in Poland. I suppose they would try and do the same on a larger scale.
How do you identify them in the midst of the population, when some of them are in the forest or dying of typhus?
Not all of them. For the rest, although on paper the criteria would be racial alone, in practice I expect they would be a mix of racist pseudoscience, political reliability, and economic usefulness.
How do you place them in some sort of working community?
Don't understand the issue here.
How do you 'Germanise' them?
Forbidding use of original language, putting the children in German schools, atomizing the survivors among German settlers, dealing draconian punishment for attachment to original culture, I suppose. Given the extreme circumstances, I assume that many survival-minded Slavs that are given a chance, would leap at it.
How do you respond to their resistance?
Well, this is easy, given the record: overwhelming brutal force.
What if turning them into serfs looks a lot easier than turning them into Germans?
In an industrial economy, illiterate serfs are much, much less efficient and useful than an population of assimilated, educated workers.
As yourself pointed out, from an amoral PoV, there was a logic behind the Nazis' actions and plans for the East, and it was to turn the area in the German equivalent of North America or Australia. It doesn't work very well if you make it a pseudo-medieval re-enactment.