Doc I use my route, it’s pretty straight forward on an iPad, prob the same on an iPhone, ut not tried it on that.
Just make a route, using which ever layer you find comfortable with, save the route, name the route, then it gives options to save as a GPX or a Tom Tom file etc. I think the crucial point is saving it to the GPX1.0 NOT the new GPX 1.1 as quite a few have had issues in doing that .... good luck