Rib caps (also cap strips) are cut after the wing is totally built and the appropriate sheeting applied, cap strips are cut from the supplied sheeting material or what is left over after sheeting the wings center section, leading and trailing edge, if it applies to this model, they are cut to size to fit tightly between leading and trailing sheeting and are glued to the top and bottom of the ribs, make sure the rib centers well on the strips, which for that size airplane will be about 1/4" wide, this creates a "T" shape on top and bottom of ribs, adding tremendous strength to the ribs and the entire wing. Manufacturers sometimes supply 36" long strips of balsa sheeting, either 1/8 to 1/4 width to be used as cap strips material, if not in the kit, use leftovers, make sure they are the same thickness, do not cut them all in one shot, do them one by one and use a very sharp # 11 blade, slow CA works very well as gluing material.