the frog that taught naruto and the snake that taught kabuto would be the teachers that teach you sage mode, the skill has two variants, the snake and the frog, both are the same (they unlock new forms of sage mode), with the The only difference being that each one is required for a different sage mode special ability.(Frog Kumite: toad / Inorganic Reanimation: snake)
To transform you need the sage mode skill at least level 1, activate sage mode in action menu, select the sage mode icon and hold G_
While holding G a hub appears which shows the percentage of natural energy/energy and physical/spiritual energy, the former which increases by itself.
If % of natural energy is greater than the minimum and less than the maximum required when you release G you will enter one of the sage mode transformations.
Similar to Mystic, Sage Mode would not have a normal chakra cost, instead it empties your senjutsu chakra passively.
The amount of senjutsu depends on which sagemode transformation you use (the more powerful the form the less senjutsu chakra) and how much chakra you had when you activated it.
While you are in sage mode two things happen, a bar that shows the amount of senjutsu chakra you have right now.
the cost of your jutsus is divided between normal chakra and senjutsu chakra