Easy enough (ofc, learn rasengan first and THEN use this jutsu in a battle <_<), ocncentrate katon chakra on either hand and then make it come out in the form of gas as you make it spin clockwise or counterclockwise (depends on your hair xd) while adding further and further chakra. Once you have reached a necessary quantity of chakra, simply make it spin more powerfully as you add a final coat of katon chakra which will ignite the insides of the rasengan and make it into a round maelstrom of fire. Thrust it onto my kankuro-looking clone.