Hello, I'm new to bonsai and plants in general, but I managed to germinate one japanese maple and 2 apple trees this year, now the saplings are about 6 inches tall. I live in alberta and it gets really cold in the winter so they'll have to be inside. How should my set up look like? What do I need to keep them warm and to have them survive untill spring? Also what should I be fertilizing them with right now and when they're bigger?

When winter comes and the leaves have fallen, put them in a place that does not drop below -10c. Most of winter they should be fine outside, just during real cold protect them.
Do not keep them indoors at room temperature or they will just wither away.

A bonsai professional in Rochester, NY, moves all his trees into a garage in October before it gets too cold and leaves them there until March or do. He has a small heater that keeps the temperature no too far below freezing, probably about Leatherback's range. He has a major problem with mice nibbling away at bark.

Your climate is probably a tad cooler than New York, but you would probably do about the same. Your trees want cold weather to go dormant. They don't want sudden extreme changes.

GurmehaqueKaur wrote: A Japanese maple should not be overwintered inside a warm house. Keep the plant outdoors, but it can be given some winter protection. If you are growing Japanese maples in pots, keep in mind that potted plants lose one zone of hardiness because of the cold air circulating around the pot and the plant's roots.

You have read the question and seen where the OP lives?