The way you're doing it is much better for words like время -> времени, the way I'm doing it, that is an exception that I would have to look up (after making a mistake). I believe that ие in accusative plural is N or G (animacy) , and the inst plural is in fact иями (бессмертие - Wiktionary). As for the ия thing, I still don't quite know which or why, but the ия (rather than just a я) is for places/countries. I saw someone incorrectly say "руссия", and a native speaker responded that that meant "land of русс', so. I think ия and ие is also used with abstract ideas, or things that aren't a physical object? That's the one I don't understand. I find it's easier(perhaps lazier) to, rather than try to memorize them by sitting there and looking them over, to just make said master sheet, and keep a copy of it on every mobile device, and always have it when you need it, eventually it just sticks from being right there for so long. Riding the bus or something and think of a word that you want to decline? Pull out the list. That will give that specific ending an experience, and an example for you to easily remember it. Like if you were just thinking, and wondered what the genitive of дверь is, you look at the sheet... and figure out... двери!, that work makes things stick a bit more for me at least.