Find or purchase a hardwood beam approximately 50 to 80 inches long and about 6 to 10 inches across/deep. The size of your beam will depend a lot on the ultimate size of your oxen. You want the beam to be snug enough to hold the animals heads well without hurting them.
Carve your rough rectangular beam down to a basic shape. The original beam will probably have been rectangular in shape. What you want to to is use a chainsaw or axe to round out its two edges. you also want to use either of these tools to roughly round out the corners of the beam. Also, with the axe, cut two large, wide notches in the beam, one near each end. These will be where the beam will rest against the necks of the oxen.
Refine your carved yoke. use an adze to smooth out the rounded edges until they are fairly smooth. Then, use your adze and possibly your hatchet to further smooth and round out the depressions where the necks of the oxen will go. Once your beam is finely smoothed, sand it down to get rid of splinters.
Use a large power drill with a wood drilling bit to puncture four holes through your yoke. Near one edge, above the area where you've grooved out the neck depressions, drill two holes right through. Base their width on the width of your oxen's neck. Do likewise above the other neck depression. You want these holes to be about 1.5 to 2 inches wide.
Find or cut two lengths of bow stock for your neck braces. These will be 50 to 70 inches long and rounded. They must be cut from the outer trunk of a healthy hardwood sapling in order to be strong enough to resist breaking. You want these two lengths to be about 1.5 to 2 inches wide. Steam heat them, and then bend them both into a U shape,tying them off that way using strong rope. Bend them so they correspond to the width of the spaces between each pair of holes on each end of your yoke.
Drill a horizontal hole into one end of each of your U shaped bow stock pieces. Carve a wooden peg which will slide through this hole and snugly fit within it. Insert the bow stock pieces into paired holes on each end of your yoke and notch them so they stay in place. Your yoke is basically done. Test it on an ox and hope you've measured right so the animal doesn't kick you for choking it.