Well katon is one of the basic five elements, from what i know using katon is the basic of all uchihas, to use katon u must heat up ur chakra in ur lungs (if the katon is used with spiting it from the mouth ex. Dragon ball tech) if not u concentrate the chakra in the limb, arounds heating it as much as u can and releasing it. Katon is weak againts suiton and strong againts fuuton, u can combine katon with mud earth techs to make flaming mud balls as the 3rd hokage made in his battle with orochimaru, or u can combine it with wind (1st the katon tech cuz it's slower) than the wind tech making a giant wind/fire combo. Btw sry for the rate reply been super busy in rl : (