here are some tips:
don't go nuts. if you separate each wire, you'll end up with a mess. just cut ever few wires and stack.
remember that the floppy cable has it's own "twist" so, work with it, your outcome won't be as smooth as the ATA cables, but it'll work
DO NOT wrap the entire cable with electrical tape. the cable will be very stiff, and fairly useless.
if your existing cable is just long enough, don't do the modification, you'll need slack because the rounding will decrease the "length" of the cable some.
use a sharp knife, preferable one with a thin blade, like a razor.
make sure your work area is well lit, and if you have a magnifying setup, use it.
ignore the problems with crosstalk, it's negligible if you stack.
make sure you do have extra cables lying around if you cut a conductor, you'll need it.