find the right diameter can be tricky but a tent of that age it will probably be 8.5mm, cut it to right length or a little longer if tent has stretched alot, file end smooth, then the fun bit, new bungee threading, start with the section that has no ferrule, that way you have to thread ferrule first which is easier on all other sections, tension up and knot it, honestly its easy, all you need is a junior hacksaw and a small file,
as to where to get a section from, tent repair services in musselburgh, or some good shops actually keep spares, and will do it for you, <subbtle plug> my local does it, its that one in stony stratford that lots of people on here talk about
