I've read that once the leaves fall off your trees take a cutting from this years growth. I've heard conflicting ideas on what to do next. I have personally started them in water, some say the roots will be too soft, I have had no issues with that to this point. 0nce you have roots carefully move them to a growing medium. You can start them in a growing medium keeping them moist and in a location with some indirect sunlight. I have several japanese maples started now. 3 are in water 2 in a starting soil

Google 'free plants' and the info there should be very helpful.