The quickest way is to plant it in the ground and let it grow (don't prune it). When you are happy with the girth of the trunk, then you can chop it down and begin training it for bonsai.
You may find this helpful too: (Its about creating a shallow root system)
http://www.bonsai4me.com/AdvTech/AT...pformbonsai.htm
Plenty more tutorial information on the bonsai4me site too.
However, this tree likes hot temps? It would be better to grow natives, like hawthorn or spruce, they may do better in our climate