The Importance of Bhasma is mentioned in Shiva MahaPurana. There are two types of Bhasma. Maha Bhasma and Swalpa Bhasma.

The Importance of Bhasma is mentioned in Shiva MahaPurana. There are two types of Bhasma. Maha Bhasma and Swalpa Bhasma. 'Shrota' (listener), 'Smart; (rites according to the smritis) and Laukik (worldly) are considered to be the Maha Bhasma. There are many types of Swalpa Bhasma. 'Shrota' and 'Smart' are meant only for the brahmins. For the rest of the castes, 'Laukik Bhasma' is appropriate. A Brahmin must apply Bhasma, only after initiating it with the mantras. The ashes which remains after burning dry cowdung is called Agneya Bhasma.
Applying 'Tripunda' on the forehead with the ashes after the completion of Yagya is auspicious and Japa should only be performed after applying tripundra on the forehead. ‪One Who worships Shiva daily and applies Bhasma on Shivlingam is freed of all his sins. One who does Penance without applying Bhasma may not be successful. One should apply bhasm on Head, forehead, ears, eyes, nose, mouth, neck, two hands, elbows, ankles, navel , chest, heart, waist, thighs, knees, arms, legs. It's said that Bhasma should be applied on Total 32 parts of the body. If One who Cannot apply Bhasma on 32 parts of the body, then he can apply on forehead, cheeks, Neck, Shoulders, elbows, Hands, Heart, Navel and Back.
One Can apply on head, Shoulders, heart, navel and ankles. And if one can't apply on these 5 places then he should atleast have 3 lines on his forehead. That is tripundra. One who doesn’t know what customs need to be followed can just apply Bhasma by chanting “Om Namah Shivaya”
All the sins commited knowingly or unknowingly are washed away by applying Bhasma on Shivlingam and by making tripundra on ones forehead.
One may have been most wicked person , but all deities will be pleased if he applies Bhasma. Devas are not pleased with the One who criticises Bhasma.
One should always Worship lord Shiva With Bhasm and rudraksha, and also should wear these two while worshiping. If he doesn't Worship , even though if he just wears Rudraksha and applies Bhasma is considered big devotee of lord and Shiva is pleased. One Can Just wear rudhraksh and Apply Bhasm and Chant Panchakshri Mantra “Namah Shivaya” or Shadakshri Manthra “Om Namah Shivaya” and Lord Shiva will be pleased.
Om Namah Shivaya ~ Jai Shri Mahakal !!