Sage Mode is the result of using natural energy along with a ninja's normal chakra in perfect balance to explosively increase a ninja's abilities.
To learn senjutsu, one must first learn to draw in the natural energy around them by remaining perfectly still. By stopping their own flow as a creature, they will begin to harmonize with the flow of nature. After feeling the flow, one must then learn to balance the external natural energy with their own chakra. Once this balance is achieved, they will enter a state called Sage Mode, allowing them to use senjutsu.
The fastest method of learning to balance the three energies is applying the Secret Toad's Oil from the Myobokuzan Mountain onto one's skin. It can help the user feel the natural energy around them before they learn to remain perfectly still. However, there is a great risk in implementing the oil and drawing in natural energy. If one draws in too little, Sage Mode cannot be achieved. If one draws in too much, they will turn into a toad. If one attempts to continue the senjutsu training and fails, they will turn into solid stone. They essentially become a part of nature. Furthermore, moving while drawing in natural energy causes an uncontrollable inflow of natural energy, thus initiating the Toad Transformation as well.