I collected some birch seeds one month ago (in August). Now I would like to plant them and grow my very first bonsai tree. What should I do exactly? Should I put the seeds into the refrigerator for cold stratification? How should I do that? Three or four weeks will be enough in the fridge? After stratification what should I do? What should I plant the seeds in, if I can't get special bonsai mixture?
Welcome.
You are asking for a guide on planting seeds. There is plenty of information about this subject on the internet, please do a search.
Based on MY experience, in MY location, I would wait to sow them by the very end of winter/beginning of the Spring . If your place is cold enough, they don't need stratification and can be planted on the ground. If your location is tropical them stratify them on the fridge for 40 days.
Soil? Any soil, even sand can make them sprout.
But don't take my word for it, these are your seeds and you should research a lot before doing anything wrong with them.