Bra wire casing is a specially made tape, with a channel to insert your bra wires.  This tape is dense enough to not allow your wires to poke through.  The casing in this section is professional grade, and heavier than what you would make yourself.  It should be applied in the same way as it would be in a factory.  After making your bra and sewing on the elastic, the wire casing is the last tape to be applied.  It runs up tot he underarm, and you simply cut it off at that point, and bar tack over the edges to prevent you wire from coming out.  Remember: bar tack one end, insert the wire, push it out of the way, then Bartack the open end.  Be very careful not to hit your wire as your needle may snap.