Figure 1: Wooden and Iron Wire Cranks.
The biggest design factor is the cross piece between the two handles can't be too long, or the cranking motion is awkward. If the cross piece is too short, you won't generate enough torque to twist heavier cords. Very small circles are also awkward. Four to six inches is a range that works for me.
The hook opening on the two cranks in Figure 1 face opposite directions. So you hold the wooden crank by the back handle so it hangs with the hook up. But you hold the wire handle by the front handle. Neither is better. It's just one of those small things you learn by doing.
Figure 2: Wooden Crank.
The cross piece should be a split resistant wood, like elm (Ulmus spp.) or osage orange (Maclura pomifera).
The handles need to take a smooth finish, without splintering.
To make the hook, drill a hole through one of the handle pieces, perpendicular to the grain, about one inch down from the end. Saw about 1/3 the way through from opposite sides of the handle, about an inch below the hole. Split off the extra wood, down to the shoulders, then cut out the notch.
Figure 3: The Hook End.
Drill two holes in the cross piece, slightly smaller than the diameter of the handles. Trim the ends of the handle pieces to fit snuggly into the holes. Cut a slot in the ends of the handle pieces, as deep as the cross piece is thick. Glue and wedge the handles into the cross piece. Make sure the wedges are perpendicular to the cross piece's grain. This will put the spreading force in line with the grain, reducing the chance of splitting. 
Figure 4: Wedged Handle Joint.
Figure 5: Wire Crank.
To add rotating wooden handles, drill holes down the length of the handles, and slide them on the crank before bending the hook. Or you if you've already formed the hook, you can split the handles along the hole's length, and glue split parts, being careful not to get glue on the inside of the hole. Wrapping the handles with twine keeps them from splitting (again). Any whipping like you use on the end of a rope will do.