You've got it! You count up every card you donate, and once you hit 20 you can submit to be given one of the completed or nearly completed decks. So once you've donated 20 cards to any combination of active decks you can ask for a completed or near completed deck and it's all yours to snag and go claim the mastery for!
As for knowing what people aren't collecting, in general people who haven't been able to get rid of cards from a deck will come and I'll do my best to confirm that nobody is collecting it. For the ones that are already up, I included Asobot for old time's sake and since nobody seems to be collecting it STILL, and I looked at what decks had a crapton of cards in recycled art that nobody was claiming. Those are usually good signs. I also plan on going around with trade offers later this week and I'll look for more decks nobody is collecting then. So TL;DR I keep an eye out and depend on you guys as well. :)
Re: QUESTIONS/COMMENTS
As for knowing what people aren't collecting, in general people who haven't been able to get rid of cards from a deck will come and I'll do my best to confirm that nobody is collecting it. For the ones that are already up, I included Asobot for old time's sake and since nobody seems to be collecting it STILL, and I looked at what decks had a crapton of cards in recycled art that nobody was claiming. Those are usually good signs. I also plan on going around with trade offers later this week and I'll look for more decks nobody is collecting then. So TL;DR I keep an eye out and depend on you guys as well. :)
I hope this helps!