Draw a plan for your wooden dummy, so you know the exact dimensions of your wooden parts.
Buy a telephone pole from a lumber yard. Call ahead to make sure they have one in stock. Inquire also whether the pole had been kiln dried, as this dries out any sap or moisture in the wood. Buy a section of a pole that's 9.5 inches in diameter and approximately your height.
Form arms for the Wing Chun dummy from whatever wood you choose. The wood should be pretty hard, like oak, or you can use several 2 inch x 4 inch boards pressed and glued together. Round the arms on a lathe or with a grinder. Sand until they're smooth. Tape the end of the arms so they it can be fitted to the trunk of the Wing Chun dummy and absorb some of the impact.
Drill holes for the arms with a power drill, cut slots in the mounting portion of the arms for the holding shims. The fit should be somewhat loose, with about a half-inch of movement on either side. Cut small pieces of wood to use as shims that will hold the legs and arms in place.
Form the single leg of the traditional wooden dummy in a similar manner as the arms. This step is somewhat more difficult, as the leg has a bend in it. Some people use a three-legged version of the dummy for more training options.
Sand and finish the wood with stain to preserve the wood.