A glimpse into the procedure:

The first and more complex encryption system, Code # I, is written in a notebook that Kaczynski called Notebook X. Notebook X is composed as a dated journal containing plaintext journal entries intermixed with portions of ciphertext. This system utilizes numerous safeguards, including the use of intentional misspellings and encryption errors, meaningless punctuation, nonsense words, and Spanish and German text intermixed with English plaintext and ciphertext. Kaczynski also chose to omit and add word breaks at random and use nulls throughout his enciphered text.

- Kaczynski's Ciphers

Everything you'd need if you have the will and time to give it a go:

• Decoding Procedure by Ted Kaczynski
• Notebook X by Ted Kaczynski - the encrypted journal
• The Unabomber's Codes by David Oranchak - a video guide and transcript
• Kaczynski's Ciphers by Jeanne Anderson - a written guide

Most of the decrypted messages from Notebook X were released by California University not long ago and are shown in the link above. But, there are still some number sequences included where if they have been decrypted, they haven't been publicly released as far as I'm aware.