Can I just say sohum and be enlightened?

Foolish

You can’t. You have to experience things. Slowly.
In small steps

Every step can be a challenge

Fools think they have reached the highest without going through the smaller steps. Can you become a surgeon without going through all the necessary smaller steps of knowing anatomy?
Merely seeing a little light is not enough

What are the smaller steps?

Here Yogananda gives the small steps

THROUGH [KRIYA] PRANAYAMA, THE ASTRAL BODY AND CHAKRAS BECOME VISIBLE

In the initial state, the yogi is busy withdrawing the life force into the spinal centers. When he succeeds in this work, his astral body with seven astral plexuses becomes visible to him THROUGH HIS SPHERICAL EYE OF INTUITION. The astral body is made of tissues of light condensed from astral rays, even as the physical body is made of fleshy tissues. When the yogi is able to withdraw his life force from the senses, not only does he see his astral body but he can disconnect his mind from the outer world.

The benefit of seeing the astral body is that the experience helps the yogi to ascend – to lift his soul, as the body-identified ego – from the fleshy prison. Afterward, the devotee learns how to take his ego out of the astral and ideational bodies and COMMINGLE IT (EGO) WITH THE PURE SOUL. The yogi is then able to UNITE HIS SOUL WITH THE OMNIPRESENT, EVER BLESSED SPIRIT.

PARAMAHANSA YOGANANDA,
CH VI, VERSES 3-4,
GOD TALKS WITH ARJUNA : THE BHAGAVAD GITA
Pg 595-596