Japanese Black maple is a very powerful and macho shrub for bonsai shrub. One of the most important features of black maple bonsai shrub is a powerful and distributing surface root structure that provides a balance and strength attribute of the types.
The best method to establish this expanding surface rootage (happo nebari) is to start just after the new seedlings have opened their first set of juvenile needles. Strip away the entire root structure and treat the rootless shoots like cuttings. The new roots will emerge horizontally and will be easy to train into the appropriate form.
Plant seeds at the end of winter to beginning of spring.
Left Diagram:
Take a seedling that looks like this. Notice the long taproot. Not conducive to a future bonsai.
Right Diagram:
Cut the roots off with a very sharp blade from where the green part on the shoot ends. Dip the cut end in rooting hormone and plant shoots in individual 3" to 4" pots in a light granular potting medium. Pure perlite is good as is small diameter grit mixed with akadama.
Keep the cut seedlings somewhat protected from too much direct sun until they root, but there is no need to cover or treat with too much care. Applying a fungicide will reduce loss to damping off. Do not allow the soil to dry out.
In 1 or 2 months, roots will have formed and you may begin fertilizing.