Stratification is like simulating winter for the seeds, most tree seeds need to be rehydrated and then kept slightly damp and cold for a couple of months for chemical reactions to occur inside the seed before it will germinate.
Our process is simple and produces good results. We usually soak the seeds overnight by filling the bag half way with water, this will help the seeds swell up and rehydrate. Drain the water and add about 1/4 cup of sand. If you don't have sand you can use peat moss or a potting compost if it's free of fertilizer. Add about 1 tsp of water and seal the bag. Label it with today's date and put it in the crisper drawer of the fridge (a bottom drawer). The temperature should be cold but above freezing. Check the bag periodically and make sure the medium is just minimally damp, the idea is you don't want the seeds to dry out, minimal moisture is the key. If you were to squeeze the medium it should clump but not drip. Sand is handy for many large seeds because when you're ready to plant you can rinse them in a strainer and all the sand will wash away - it makes it easier to find your seeds. Some seeds like apple can start to germinate in the bag even at 1-2 degrees C, if this happens that's a good indication that they're ready to plant. Also a good reason to check the bag periodically. If you time stratification to be finished for early spring, you can take the seeds outdoors and spring plant them. Some seeds take their time to germinate. We planted some Korean pine this year and they steadily germinated from June through August. Many will start to grow as soon the soil reaches the right temperature. Many tree seeds can handle some frost, if they've germinated naturally in early spring outdoors and some cold weather is coming they'll be fine. If you decide to germinate them indoors under lights, they'll need to be moved outside after risk of frost has passed and gradually hardened off to the outdoor environment.
This stratification method is just how we do it, and we've germinated many types of trees and perennial seeds this way - but every tree is different, and some germinate better when stratified outdoors. For sure this a topic you can get more in depth about, and take note if your seeds have any special requirement like warm stratification - which is the same as the cold version, you just keep them at room temperature. Some seeds need an extended period of time to germinate and do best if you plant them outside in a bed and just let them take their time, while others start growing right away. You'll find detailed instructions for stratifying every type of seed with a google search like "(name of plant here) seed stratification". We would encourage you to do your own research and try different methods. Starting some indoors in the fridge and the rest outside in a pot or a garden bed can be a good way to compare methods.