A good site with many details on the grafting process can be found at: http://www.ces.ncsu.edu/depts/hort/hil/grafting.html
Use a spliced side-veneer graft in winter just before bud break or in mid to late summer. Moderate success may be achieved if the scion and rootstock are from the same genus (i.e. Acer), usually the same species. Acer palmatum often has viable seeds if you were mearly interested in propagation.
It is easier if the rootstock is thicker then the scion. Two-year-old, seed-raised stocks are most often used. Be sure that the rootstock is not rootbound in it's pot, and that it does have a healthy root system.