You probably have zelkova serrata (japanese grey-barked elm), it would make sense coming in a bonsai kit. Chinese elm (zelkova sinica), and caucasian zelkova (zelkova carpinofolia), would most likely not come in a "bonsai kit," however I could be wrong. Cold stratify means that the seeds need to experience a period of cold before they will come out of dormancy. This period is usually 2-3 months, however I've succesfully cold stratified seeds for a month before. Don't try that though, a period of 90 days should be perfect. There are multiple ways to do it. You can put the seeds in a damp paper towel and stick them in a plastic bag and keep them in the fridge for 90 days. I noticed that you said the freezer, DO NOT put the seeds in the freezer, they will most likely die. While the paper towel works and is the easiest, this is the most effective way that I use. Take the seeds, soak them in warm (not boiling) water and leave them in the water for 24hrs. Take a ziploc, fill it with half peat, healf perlite, and make sure it is moist but not soaking wet, no puddles in the bag. Mix the seeds in with the perlite/peat mix and put them in the fridge, for 90 days. You can use a fungicide from the local nursery too which can keep mold from forming in the bag, which I highly recommend. If mold grows, the seeds either need to be thrown away or sown immediately. Put them in the firdge. Check on the seeds every other week, you do need to do an elaborate check just make sure mold isn't forming and that the peat is still moist. After two months the seeds might start sprouting in the bag, plant them outside. Don't expect this, because it is possible that none of them will sprout until placed into hotter temperatures. After 90 days, the seeds that haven't sprouted should be placed outside to grow. The rest should be pretty self explanatory, keep the soil moist, keep the pot in the sun, etc. I hope this helps, good luck!