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I did something like this last time for my crypteks and the reactions helped a lot so I thought I'd try it again.
So, how?

I would put yourself in a time capsule for 2-4 weeks, because after that time the GW models for them will have likely come out.

If you are really hell-bent on converting your models right now, I know of no good/easy way to go about it. They're big, complicated designs. Even the relatively simple deathscythe design is tricky, I have yet to see a scratch build of it that looks good at all.

I would wait, but there has been no confirmation of a 2nd wave.

how about trying to make a cresent moon for start, and applying the necron touches as needed.

As for the stalker this was the best wip i have seen
http://www.dakkadakka.com/gallery/304412-.html

What you use to convert your models depends highly on your resources available to dump into this conversion.
With infinite money, I'd use the back pieces from two Ghost/Doomsday Arks as a base for my Scythes , adjusting where they connect in the middle until they're the right shape.
Since it has a Tesla Destructor, I'd grab one of those from a Barge kit and mount it underneath as shown in the codex.
For Nightscythe, I'd get the little back portal from the Monolith and make a small underslung portal for the troop disembarking portal.

Now, if you have little money, like I do, and you just want something to count-as for casusl games, flgs tournies, and the like: paper plates, necron bits, black spray paint and some glue/tape would do nicely.

quick off topic but what about these as Tomb Blades?