I am talking about a leather-covered wood scabbard.

You want to use lathing, cut somewhat longer than the sword blade. You need two pieces, one for each side of the sword. Lay the blade on the lathing and trace the blade's' shape, then carve out the lathing to make a little hollow where the blade will sit. Do this for both sides, then glue the two sides together w/wood glue, use clamps.

smooth the outside edges of the scabbard, and shape the bottom of it appropriately (usually a bit pointy, depends on the style).

You could always quit here and paint your scabbard and be done.

You could also make a linen wrapping, in the old Saxon and Frankish style. see http://www.regia.org/sword.htm

For leather, you can cut a piece of leather to fit. If it is veg tanned, then wet it and stretch it around the wood. If chrome-tanned, use ordinary rubbing alcohol. It wants to be very tight, cut off the excess You will sew it up the center back side, like you do a knife sheath.

Adding chapes and such is optional.