i'd say the same way you would start it with another human... you just don't use words to communicate. sometimes you initiate, sometimes they do. i don't think there's a single way to start.
my girl more or less initiated during her second heat (tho it is debatable if she maybe just interpreted me massaging her as me initiating), my late boy found out on his own how to get his sheath into my hand during a petting session. with my current boy, i was the one to show him that a mere touch in specific spots can feel really good. i just went lower and lower during belly rubs.