Japanese maple is fairly easy to grow from seeds provided you remember 2 things. One: you must have fresh seed. It does not keep. So if you have seed from the tree you must plant it now, not in the spring. That's right - in the fall. The seed will not germinate until it has passed through several months of very cold weather (stratification) which reflect its natural Japanese climate.Sow in pots in ordinary garden soil and cover with gravel. Don't forget to label them as you have a long wait ahead and might forget about them! One final small observation. If you have one of those fancy cultivars with purple leaves, the seed will not come true ie. not be like the parent, though you make get some interesting variations. Good luck. Ian.