@craig-spencer You can choose between the TomTom format called itn , or you can use the GPX 1.1 which i think is the same format in use by Garmin.
Most people here give preference to GPX stating that a GPX format forces the GPS to exactly follow the route you have prepared while itn gives the GPS more options to deviate in favour of its own calculations.
In my experience putting sufficient waymoints in an itn does give also good results.
Myself, I doubt what is best. Just try thme both and see what suits you best.